School problems.

N🌻
So I'm currently a year 11 in high school, like 70-80% of the time I struggle to fall asleep. I literally do everything I can to fall asleep, been dealing with this for years. So it's nothing new. Sometimes I can't wake up in the morning and am physically too tired to even more. I get to school at 9 the latest and usually only miss just home group. Nothing too important. I'm not behind in any class's, I'm not failing. I'm on a B average. 
Sometimes I sign out for family reasons because my mum has a doctors appointment and no one can watch my two year old brother. This happens one or twice a month if that. And I only leave school about half an hour early. 
Today a teacher comes up to me and says I need to talk about why I sign out with family reasons and why I'm apparently always late and miss so much school.
I've had a total of five days off this term. Was sick for three of those and the other two were sports day and a study day thing. 
I told him the reasons and he wouldn't take it for an answer. He then went and suggested I get high to help me sleep. I'm agaisnt it because it can trigger a mental illness for me and I'm not willing to test it. 
So since they keep hassling me and won't take my real reasons and even suggested drugs (which is illegal where I live) can I do something about it? And get them to back off?