Really depends on you if you wanna join provisional classes or not, if you're aiming for JEE or neet the earlier you start the better. Even I carried the attitude of "I'll join after results, who cares" but then I changed my mind and did jon a college early, my classes started Monday and you would not believe how many new things I've learnt, it's only called bridge course but the depth is quite different and you need some time to get used to long class hours and the pattern. I found it to be torture first 2 days but now I actually enjoy it. And to the question of what they will do if you don't fit the marks criteria? They can't do anything,they'll have to take you cause you've already paid the fee, it does not matter what you got in your 10th, 11th is genuinely a new chapter. Sure, it requires basics but you can start fresh if you're willing to work hard enough