There is a common misconception that all skiers and snowboarders etc. are dumb and have bad grades. Personally you can choose not to give a fuck about your grades and just focus on skiing. Or, you can have a balance between the two which is what some people still need to learn. If you have a day off and can go skiing but have a big test or exam the next day, maybe go skiing for a bit then study. There is a way to do both.
Oh and go tell you teacher to go fuck himself because you are able to balance grades and skiing!