Just do what every shy geeky teenage boy has done, wear a lose shirt if you can, take off the shoes, grab the pants your going to put on keeping them near by. Drop the pants and put the new ones on right away so the shirt helps cover things and your out of pants for like 10 seconds.
The shirt can be more tricky. A binder that's like tshirt especially if it looks sorta like under armour and like the pants keep it on hand, pull the old one off and pull the new one on. Do both whail facing the wall and don't talk to anyone whail actually changing. Odds are the most anyone will think is that your shy about your body which is true in it's own way.
You could come up with a elaborate excuse for the under shirt like a horrible burn that has to be kept clean, but guys may want to See it and its s lie you have to stick to long term, best to go simple, something like "I sweat, it helps" short simple lies are the easiest to believe.
Serena