Our whole school is this mixed grade system. Not bad at all.

The only drawback I believe is that the teacher can't concentrate for one cirriculum, she will have to prepare for two. Less time and effort for one level and more interruptions than regular system.

But for elementary school, it is more important to culture the people skill and learning habits than academic achievements.

My kids love it. The best part is that every September they alway know someone in their new class from previous .