Left crank keeps coming loose Often when I go to school on my bicycle the left crank arm starts to loosen up after one or two really gentle jumps. When this happens I just tighten it with the tool I have in my backpack. It went on like this for almost two weeks after which after only one kilometer of riding it starts to creak very loudly and it also loosens up. 

When I go to the repair shop, the worker just tightens it up and says it will work but after only one minute of riding it, it loosens up again. Please tell me how to fix this problem.

It is a square taper crankset where the crank is held on by a nut.
I’m assuming you have a square taper bottom bracket + crank arms?

See this answer: Square taper crankset develops play after changing bottom bracket

If you’ve been repeatedly riding with a loose crank arm it could be that it’s now damaged.

You could try tightening with lots of torque, grease on the square taper and threadlocker (e.g. blue Loctite, but in a pinch any kind of glue works) on the threads. A retaining compound (e.g. LOCTITE® 648) on the square taper is even better than grease.

If that doesn’t help, the easiest, surest solution is to replace the bottom bracket and crankset. A completely new Octalink crankset (which doesn’t have this problem) can be had for less than 30€ (+ bottom bracket for &lt;10€).