We received a notice about preschool registration for next year. We're lucky that we found a twin-friendly preschool. I know you're thinking that all schools should be child-friendly, whether they are twins or not. It's really not always the case, though.
Many schools have a "philosophy" about twins. Sometimes the philosophy puts twins together; other times it forces them into different classrooms. More than one twins Mom has complained about being given one parent-teacher conference slot for her two children. It happens more than you can imagine.
The thing to remember is you know your children better than any school district. If you want them to stay together and the school district wants to separate them, you can fight the decision. (Or visa-versa of course.) It's your decision, not theirs. And, many states have passed laws or resolutions stating that school districts should work with parents, and that in the end the parents have the final say about how to best place their children.
Twins Magazine has a series of special reports on schools and twins. You can also search the web for other research on the topic. It's important to do your research and be armed. In the end, remember that you can fight the system and win. Your children's education might depend on it.