Personally I do cover up scars always socially and at work except when at home. I tend to wear thinner sleeves, there were some shirts in Next including in the sale that looked like blouses but the sleeves were sheer.
I worry about showing scars in case people ask me about them especially at work or that they (wrongly) make an issue of it/talk to others about me. I would be honest though I am not sure of their responses, am not always feeling robust enough to take comments.
For you it sounds like you are used to wearing short sleeves so there may not be reasons to change that around teenagers, and the activities will be very physical ones so it's appropriate. What is your role there and are you with a group that consist of employers/teachers who are assessing you with these teenagers? That again may influence your decision.
This group of teenagers may not notice, or be relaxed or feel for you (any or more of these reactions). It is a personal decision and okay whatever you go with. You should be proud of who you are and the great plans you have in terms of teacher training, you must have come a long way.
Interviews though for sure, best to find sleeved clothes then you can always take more 'risks' once your employer knows you and depending on context.