When my daughter left the hospital, I didin't even consider treating lows. She was so high, that it seemed impossible that she would ever be low. She was dxd in March, and of course by now we have treated lots of lows. It took us about two weeks to get our daughter in her "normal" range. Soon after, we started dealing with lows almost daily, and often, twice a day at school. The most important thing is to have a medical plan or 504 in place, before school starts. I would set up a meeting this week with the principal, your sons teachers and the school nurse. That way, you can determine if, when and how you want to be notified of his lows. My daughter is 10, so I have the school call me anytime she tests her sugar. Even if it is normal. It helps me to see a pattern in her highs and lows.
The good thing is that my daughter is very good at feeling lows. I think most diabetics are. Especially once they have experienced a few lows. It makes it pretty easy for them to identify the feeling. The thing that helped my daughter the most was to let her know that it is ok to test her sugar whenever she feels like she needs to. It is not something to be determined by the teacher, but by your son. The more he tests and recognizes the feelings of highs and lows, the more confident he will become.
I remember taking my daughter back to school in March, and believe me, I was terrified. But sooner than you think, it will all become part of your "normal" life. Communication with the school is so important! Don't ever feel like you are a nuisance. The school has a responsibility to provide care for your son and part of that care is communicating with you.