How much liquid can the bladder hold before it bursts? and also, how do we measure the amount of liquid in the bladder in an 8th grade science lab?

Depends: Normally, you would like for the bladder to hold about 3-400 ml of urine, but it can hold much more than that. I have seen patients with a 1000 ml or more in their bladder, but they are usually in pain. For your 8th grade class, I would ask the local hospital, ER, or clinic if they have an ultrasound device that could be demonstrated for the class to show the urine volume of a bladder. Good luck.