There is a list of all missions with guides at http://bullygame.wikia.com/wiki/Category:Missions
You can still get the random missions once you've completed the story and enter the 'Endless Summer'. So if you've finished the game, take a good wander around the map at different times of day to mop up those last few.
This achieve took a long time to pop. I completed the last mission and had 76 completed showing on my stats, but no achieve. I ran around for a while collecting bands, cards and gnomes waiting for another star to appear on the map.
Eventually, after about 45 minutes, the achieve randomly popped while I was walking around the school. So if you find yourself in the same situation, don't quit till it pops.
From the comments below, it may be worth trying to pop this once you've done all the missions by saving, going to the dash then reloading the game.
Worth a try if leaving it running doesn't work.