you can quit (go out) and then withdraw, after withdraw change in SR and ttRating is applied,
however if any1 else joined same session with you SR and ttRating is changed after 30 min, when session ends
btw if you are working on your SR you should know that fastest way for it is to race because for race it is factor 1, for qualy 0,5 and TT factor 0,3, meaning that in TT you have to do 3x more laps for same SR achievement