I'm starting at a new school tomorrow. I'm doing normal gym class and using normal male locker rooms. I pack everyday anyway, but it's extra important that i pack on gym days since im using the mens locker rooms. I usually hold it in place with the " DIY simple harness" or whatever it's called, its the one you make out of two peices of elastic. When i run, it kinda drifts up and looks kinda bonerish. I'm not afraid it's gonna fall out, but i don't want to look like i have a stiffy in gym class. Is there something I can do to make it stay where it's supposed to? A different kind of harness maybe?

Jockstrap

Jockstrap is probably your best way to start.  You plan on keeping it on all the time I expect.  Do you have to take showers, that would present new problems.  I have able to take gym classes in college where you don't take showers, golf is one.

Thirding the jockstrap idea.

Definitely either a jockstrap or you could even try poly-pro/Under Armour-type shorts, which I've found to be pretty effective holding a packer in place. Not to mention, with the right ones you can feel cooler, which is nice for gym classes, and they wick sweat away. I imagine when it comes to just basic P.E. Under Armour-type shorts are more common than jocks as well.

Though, I suppose you could solve that by wearing your jock under whatever your underwear preference is...but for the sake of options I thought I'd mention the shorts.

If you don't mind being a bit hot:
Jockstrap or harness with leg straps
Underwear (boxer briefs)
Compression/lycra shorts
Then your normal shorts

This is my combo when I am doing things like search and rescue training where we are in climbing/safety harnesses and running round and climbing over and through things and I have never had the packer move.

My first thought was "jockstrap." Clearly, I am not the first to think of it!

I don't have a jockstrap, so I've used medical tape before to kind of tape it on really well.

jockstrap the type with a pouch to hold a cup for sports. so there is little to no chance for it to fall out.