When our DS8 skipped second last year, we told him after the school year. The thinking being, we didn't want the onus of explaining to the other kids (or teacher) to be on his head at the end of the school year. Since there is always churn between years, that seemed like the best time. Kids he knew outside of school (that he saw in the summer) he could tell as he wanted in that smaller context. It is a small difference that it is the same school.