I already bought the panties from the cheerleader, so why is the task stuck on my chalkboard?

Did you by any chance purchase the item before it appeared on your chalkboard? To trigger the XP and the removal of the task from your chalkboard the task must be done *after* it appears on the chalkboard.

Here's how you can fix this:

1) Bribe Malone with the panties. As long as you have the panties on your wall. You can either ignore him while he's at the door and he will take them on his own. Or, you can tell him to piss off and instead of raiding your stash, he'll take the panties.

2) Once the panties are off the wall, you can buy another pair from Sandy. That should trigger the XP.

Remember, as long as the panties are on the wall, you have that layer of protection from Malone. Once they're gone, you need to buy another pair from Sandy!