I’m often asked about the International School experience and how it compares to that of US public schools. Mostly my answer is: “It’s great!” Our boys like it, and we do too. There are a lot of pluses, some of which I’ve written about in other posts:

Exposure to classmates from >50 countries and all the learning that happens between the lines because of this
Above average, nationality diverse teachers with teaching experience usually in several countries
Smaller class sizes
Infrastructure and resources for often underfunded US programs like music, art and language
All are welcome invitation to try -- be it art, music or sport
Integrated curriculum
Classroom discourse of international not just national events
More in class work, less homework
Less grade inflation (particularly in high school). A 7 (an A+ equivalent) is a very hard mark to earn.
Single campus for Kindergarten through Grade 12
Healthy, tasty school lunches
Earlier independence (overnight trips, homework not on a parent accessed portal)
More tolerance/inclusion of all cultural and faith traditions (school isn’t boxed into generic “Happy Holidays”)
Every student is a “fish out of water” as few are living in their home country

However, like everything, there are also some minuses. Not that they were hidden before, but it takes time to notice and feel the impact of the less than perfect parts. I can only speak to the International School of Luxembourg which may or may not be representative of all International Schools in Europe and I can only speak from a filtered parental lens, but for those considering (or dreaming) about doing something similar, a few (I hope relatively objective) things to keep your eyes wide open:

Language fluency takes more time than you expect. The entire curriculum is taught in English. Kids take language classes 4x a week (with <10 students) but it’s still more exposure than immersion. At the younger grades many kids are still learning English as a second language. Once kids have mastered English, they move on to French and then French AND German starting in Grade 6. You can augment with language tutors (and many people do), but your kid will not be speaking fluent French in six months. Even so, with classroom instruction 4x a week plus the opportunity to use it in daily life, it’s not something that’s easily replicated in the US.
Only “in the box” children need apply. For whatever reason, the International School here is unable (or unwilling) to handle children on either end of the learning spectrum. While they do have a lot of ESL support, they don’t appear to have support for Special Education or Accelerated Learning. There seems to be no latitude for a gifted student to take any classes above their grade level, nor do they act particularly comfortable with ADD or other “special” kids (especially medicated ones.) It could also be a reflection of supply of demand as there are more families wanting spots than are available and so the school may weed out outliers.
Slower progressive academic rigor. Generally speaking, academics (math being the most referenced) seem less rigorous than the US experience at the lower grades but more rigorous at the high school level (likely because they teach to the well regarded International Baccalaureate program.) My understanding is that middle school ramps up in intensity making it more equal to the US (though we don’t have personal experience there yet.) Some American families I know do augment with tutors/at home work to keep their kids at US math standards to help with re-entry on the return. With the lighter homework load, this is fairly easy to manage.
The wealth factor. This one is the hardest. The money at the International Schools is outrageous, and it is sometimes flaunted or used as social currency. It’s most acute in high school. So while there is a dynamic cultural diversity, the lack of economic diversity is a problem. American kids (at least at our school) are considered the “least well off” on the grossly skewed economic scale. This one makes you want to scream because it’s such a lopsided bubble – a bubble your kids can’t always see. You will not like some of the things to come out of your babes mouths ....
Your child will lose a BFF. Kids come and go from International Schools all the time and that fluidity will cause some heartbreak. The good news is that friendships build fast and new kids are warmly received. Most kids remember being the new kid not so long ago which elicits an institutional hospitality. While some kids do gravitate towards those from their home country, by and large friendships cross pollinate. These multicultural friendships are priceless.
Separation of sport and school. While both competitive and club sports are offered at the International Schools, it is more participatory than competitive. If you child is a driven athlete, you will need to seek out out-of-school club sports programs. Sports aren’t offered in the local schools so all the talent is pooled in the club programs. It’s not a bad approach really and we’ve adapted to it just fine. In truth, this is a net positive because it forces you out of the international bubble into the local community. Both of our boys play on club basketball teams with all local kids where the instruction/coaching is in Luxembourgish or French. This could also be true for the Music and Arts area … I just don’t have experience there. In general, whatever extracurricular passion your child has will need to be stoked outside of school.
Recess = football. Football (American soccer) is the International language of the playground and for boys not into chasing that white ball recess can be a little intimidating. The non-Footballers will find their tribe; they’ll just have to move swiftly to get off the pitch. (In Grade 1, football has been disallowed on Tuesdays and Thursdays much to the chagrin of my son but surely to the joy of others.)
Parents not needed in classroom. No offense to you, but parent volunteers are only needed for holiday parties and some field trips. Teachers are resourced with assistants and there is not the same culture for having parents participate during instruction. It's not that you aren't welcome, you'll just notice that you aren't encouraged in the same way that many US public schools do.
Social growing pains. With so many different cultures represented there are many shades of grey when it comes to appropriate behavior, language, and social norms. This requires a lot of patience on behalf of the school and students to understand each other. In the US schools are quick to intervene in social dynamics and the “bully” and “victim” label get slapped on quickly. Here they give kids more rope to try to solve issues on their own. When intervention is required they handle it internally without overly involving the parents. They also do a good job of labeling the action, not the child. This can be uncomfortable for children in the short term, but as a parent of an impulsive child I appreciate the trust they put in all kids to “work it out.”
No school bus. There are two free school buses to park and rides in the city and a handful of paid vans to outlying villages. Many high school kids take metro buses, but most parents of younger kids are expected to drive their children to and from school. This can be difficult for dual working families (although the vast majority of families at the International School have one parent at home.) Thankfully my kids are able to take one of the gratis school buses. I say “thankfully” because there is an inefficient single lane of traffic funneling into this large multi-school campus ... which I will not speak of now.
All the resources in the world can't make an EZ online school calendar.
Katy Perry is big here too. They still listen to all the same Top 40 Pop music as they do in the US. On repeat. Fifth graders will be listening on their iPhone5s.