Hey there Yash , there is nothing like registering in colleges when you are in 12th grade, but if you wanna apply for an entrance exam, you should do it before the deadline, for instance, the application for NEET UG commenced ny november 1 2018 and even if you are in 12th now, you should have applied for it before the deadline of december 7 2018, hope this gives you a view, good luck