In teenage years such things can happen as if it got a mind of its own. It means you're healthy and that part is working fine. When you are hard, peeing is very difficult.
What have worked for others is:
A: Spread your legs and push out the pee as if you were taking a number two but instead of the number two muscles you use the number one muscles (lowest stomach muscles and .muscles inside). Basically it's muscle control, something you have to learn. When you can pee make yourself aware of what muscles you use and explore stopping to pee and then begin peeing again. Control the stream, slow drip to really pushing the pee out.
B: Sit down and pee.
In combination to one of these methods you can learn relaxation techniques eg center your mind or clear it and take a deep breath. Continue concentrating on the pee muscles, focus on trying to pee. Stay calm and take deep slow breaths. Learn to relax. Turn to wobbly jello.
Remember patience. Learning takes time. While you can achieve the above without it, yoga is a good way to learn muscles control (and it's good exercise). Just don't go for the new-age nonsense "yoga".
This should go down as you age out of the teenage years. As you are just 14 it can get worse since you have just entered teenage years.
If the above methods do not work and you start to experience pain and serious bladder overflow, see a doctor or the school nurse. Having the bladder full for an extended period of time can cause other issues.
Hope this helps you.