As it's getting colder you can wear layers and roll up your jumper or coat and use it as a pillow. No one would notice that.
I have seen tens pens on Amazon. Don't know how good they are but they are discrete. Have a look and read the review.

Try different things in case it works for you.
Some things are a waste of money but at least you will know if it helps you. I’ve gone three times to an acupuncturist. Sadly to say I will not go again because the third time was not the charm. It never helped me but some people say it helps them.

Sitting too long can be a problem. I can be enjoying myself not aware how long I have been sitting. Then I try and get up and I realize rigor mortis has set in. I can barely walk.
This Thanksgiving I wanted it to be different. I used the idea that Margi_21 suggested. I applied a non smelly arthritis cream. I wore thermals under my clothes because any cold can start muscles spasms.
Most important I got up frequently. I would do simple things. Go to the bathroom, let the dog out, and stand for a few minutes. It worked great.
I have learned to sit towards the back of the room, to allow movement without disrupting.. I always carry a back pillow. You might feel self concious with the pillow, but it was worth avoiding hours of pain.
Besides no one cares if you have a pillow and after the first time they won’t even notice.
Swallow your pride and enjoy your self more. It is worth the effort.

You could try rubbing your legs with a non smelling muscle relaxer you would need to go to your Pharmacist to see what they might have.

Other than that your Tens Machine idea might be a good One.

Hope that you get lots of Suggestions that may help you out.

Being sixteen is tough without added issues. It is an age of blending and fitting in. Kids are not always kind if you fall outside the norm. Quietly inform the teachers that she needs flexibility. Sitting on her jacket was a good suggestion.