I have heard that those types of meds can affect behavior, but I guess it's different for each person. Before my son started taking meds for ADHD, taking a nasal spray with steroids actually *improved* his behavior.
When you meet with the teacher and principal you should ask if they have any teachers that are trained to help kids with behavior issues because it sounds like they don't have a plan to improve your son's behavior. My son(who has Aspergers/ADHD) is now in 2nd grade. He's always gotten a daily note about his behavior. In pre-k, he could earn a smiley sticker for each part of the day he did well at. In kindergarten, the teacher would tell me what punishment he should receive at home(such as no TV) so we were on the same page. In 1st grade, he started off with a teacher with lots of qualifications and experience, but halfway through the year she couldn't handle him and they switched him to a younger teacher. It worked out fine because she was more patient, willing to try new things, more understanding of disabilities, and the daughter of a great teacher, too. Now in 2nd grade, all the kids have a notebook that goes back and forth to school every day. If they have a good day they get a stamp, if not there's a short note. If I have a concern, or if something unusual happens at home, I can make a note on the page for that day.
My situation is probably more extreme than yours. The first half of this year was really bad. Even though the school was doing their best to help my son with an aide, work out in the hall if he became agitated, and lots of breaks, he was still having trouble. The classroom teacher wasn't that involved with the discipline- it would go straight to the assistant principal. My son would end up in the office for hours, writing long apologies(an extreme punishment because writing is very difficult for him) for behaviors related to ADHD that he can't yet control. In the office, he wasn't allowed to eat lunch or get a drink of water. It would get to the point where they would call me because he was acting like a crazed, caged animal. He would hit and spit at the teachers, hide under the desk, try to run away, lick the walls, knock over chairs and papers, talk about ghosts haunting the school so no other kids would have to suffer, and say he'd rather die than go back to school. The next day, he would be seeking the approval of these same teachers, giving them cupcakes, showing them art projects, etc., as if nothing ever happened. None of the punishments stopped the behaviors- they actually made them worse. However since starting ADHD meds after christmas, he has not been to the principals office a single time and is having mostly good days.