I probably never knew math or really forgot it

So im 15 im in 10th grade its start of school im in new big school because my old school only keeps up to 9th grade, and on math lesson we were we had a review of functions and there i understood i never really knew them, y'know my old school was small, like 3-5 people per grade and teachers often helped us a lot like giving a bit better grades or giving slightly easier tests, gived us more attention but still did their work teaching us, and in new school when they did some exercises at board i understood that I know like 40% of what they were doing at board and ive thought I dont know what theyre doing and what is this and i remember functions but we did something else, ive came up to conclusion that they are learning some other methods or smth else or I forgot or I am a dumbass, im doing pretty good and I understand it in other classes, i probably need to lock in and study or I will shine on other topics, i guess I need to do first variant, what everyone thinks?