I usually play my oboe for band, and occasionally my Gemeinhardt flute. For marching band, I borrowed one of the school's Yamaha flutes. However, whenever I put the Yamaha together, it won't come apart! The headjoint stays stuck in it. I've tried cleaning it, using cork grease, even tape (at the suggestion of my band director, strangely enough). What should I do?

use another one...most likely the headjoint was dropped and is now bent or is not a perfect circle anymore

That happened once to my old flute. I tried everything, but nothing worked. I ended up taking a pair of pliers to it.

I DO NOT recommend doing this!!!!! I'd take it to a tech if it REALLY wont come out

"However, whenever I put the Yamaha together, it won't come apart! "
If you orrowed this flute from the band director, I suggest taking it back to him so that it can be fixed properly. You could easily damage it more by trying to take it apart. Let a repairman do this.