Why does it sound like every other 4th grader has autism

This is an observation I've been noting for several years now. It seems like the average class will have 12 out of 30 students with either autism, ADHD, bipolar disorder, or obsessive compulsive disorder. Many of the autistic students are not diagnosed as "severely" autistic but are still on individual education plans and have a one on one paraprofessional with them. Has there been an actual rise in autism this dramatic?<br><br>Edit: Sorry for the lack of replies, busy teaching. I think the changes in criteria probably has a lot to do with this. I can't imagine these kids would have been diagnosed 10 years ago.