"Are their any exercises or phisical therapy that a highschool pitcher can do to avoid UCL strains?"
The UCL is a ligament (not a muscle), which means that (by definition) there is nothing that you can do to strengthen it.
RTusk40's exercise may help strengthen your muscles (possibly including your Pronator Teres if you do it right), but won't strengthen your UCL.
If you pronate your forearm (turn it counter-clockwise) through the acceleration and release phases, then some of the load that is normally focused on the UCL will be taken up by the Pronator Teres muscle. However, you have to start pronating very early on in the process, not just as you are releasing the ball.
Of course, the above will only work if you are older than 16 (and the growth plate of your Medial Epicondyle has closed).