I'm not a doctor - I cannot tell you what the issue is from a couple of lines on the Internet (then nor could a doctor) - you said the MRI showed a bone bruise - I was responding to your "if it is just
a bone bruise" They can be nasty, hurt a lot (especially if under a weight bearing surface) and can take longer than 3-4 weeks to heal. If you run in 3 -4 weeks it may well hurt
The doctor thought it is a meniscus tear, MRI showed not, but MRIs are notoriously poor at showing meniscus tears - follow the doctor's advice, and the physio and see how you go. Meniscus tears often come with pain, swelling, locking of the knee and potentially buckling. They can heal alone depending on where they are - if they are in the bit of the meniscus where there is blood supply.
At 15, you need to take the advice of your doctor, physio, sports teacher, parents.