I’m not sure if this will help, but whenever I get overwhelmed, even when I’m completely exhausted, sometimes taking a 1-2 mile walk or doing a 30-minute exercise routine will fix the problem for me. It might be worth experimenting with if you haven’t already tried it.
Can you identify what is making you so mentally and physically exhausted at school? Keeping a log of what your activities and how you feel throughout the day might help identify the problem. When you feel bad, you could write it down in the log and then try to remember what you were doing leading up to the moment when you started feeling bad. If you can identify the triggers then it might lead to a way to avoid them.
It’s difficult to answer these questions in a general way without knowing you and your specific situation.
For memory techniques: Links to Art of Memory Resources
For a quick method to help remember and understand material better: How to Use the Feynman Technique
The book Make It Stick also has good general tips about retaining information.