You cannot study effectively when you are mentally or physically tired.
When you can no longer concentrate it is time to take a break.
Be sure your breaks are not taking up more time than your studying is!
The “lights out” time is there for a reason – you need your rest. If you do not get enough rest, you cannot learn as well.
Eat right, get plenty of rest, and wash your hands regularly to minimize the spreading of germs.
If you do get sick, let an adult know as soon as possible – your fellow students are not doctors (yet) and your parents are not here.