You do not, technically, need to max out Sumire Yoshizawa in order to experience the third semester. However, you are doing yourself a disservice if you play it this way, in my opinion. It is best to max her out to help you out gameplay and story-wise in the game. Be sure to reach rank five in her Confidant by December 22 so you can access the next five ranks in January. Also, be sure to max out Maruki’s Confidant or else you will not even be able to access the third semester at all.