Medical attention is so important because they will need to check a few things with your baby. For example: Most babies come out head first, but sometimes they are the other way round, or sideways, or butt first and that can make delivery tricky and dangerous if they don't know about it in advance (if they know its OK, because they know what to do!) There are conditions where the placenta (which is what connects you to the baby inside) grows very low 'blocking the door' as it were - again, can be managed fine if they know about it, but could be dangerous if they don't know.
None of these are things to be worried about, but they'll want to do a scan just to check these things. They'll need to have a feel of your tummy, take your blood pressure, and measure how fast your baby's heart is beating.
By doing these checks, they can make sure you are safe and well.
And some people might disagree with me here, but once you've spoken to the hospital/health clinic, and have talked to your mum (or mums friend) then please try and take it easy. Most women stop work a few weeks before their baby is due, and you'll have things to organise, so I'd stop worrying about school just now if I were you.
Once a plan becomes clearer you/your m can talk to the school about whether they want to give you any work to do at home, but right now, as a 40w pregnant young woman, have a rest, and maybe get a book on looking after babies as it will help you have a bit more confidence as a new mum.