I want to be able to beat this guy?

you see there are these bully at school and they decided i am one of the unlucky people they bully. sadly they are all in my class and i have to deal with them but there is one problem for them i am tall not amazingly strong from 1 to 10 i will be about 5 and skinny. there are 6 people in my class including me and there are 3 of them one of them is tall a few centimetres taller then me and fat. that s a major advantage but i heard if a person knows martial arts well its better to be short and skinny. its summer holidays and next year i would be in the same class. i don't want to get into a fight with them i think it might be inevitable maybe in some miracle i might be able to prevent it but better safe then sorry is there a way i could be stronger then tall and fat here in 2 months some kind of martial arts? but i would like to do it from my house until my dad would find a martial arts master so is there some kind of site that could teach me oh and i am a 13 year old boy.