You will get the 5th sex point after finishing the Jessica jerk off scene. But don't stress it, even having 5/5 does not get you the next game of dare/chicken til another update.
I think there's a bug then, since I've completed literally everything else, and I still end the Ellie scene with 4/5 exp. Kind of annoying that I went through all the effort of going through everything twice, just to find a bug.
Yeah, looking at the code (line 6312 in 3_School_label.rpy), there's a variable called "added_sex_exp_af_s" that has to be 0 in order to get sex exp from Ellie. If you've completed anything that sets this var to something other than 0, you won't get XP. My first play through, I did this quest first, and ended the version with 5 sex exp, but this play through I was stuck at 4.
Sigh, would have been cool to find another hidden scene by saving Ellie for last. Also annoying that the hints say I have 5/5 for Ellie but still won't let me continue; hopefully this gets resolved in the next release. It might be more fun to have different events for different levels of sex exp; like, 1 sxp = hand job, 2 sxp = blow job, 3 = boob job, 4 = thigh job, 5 = tip, 6 = vag sex, 7 = anal sex. Keep the players coming back to Ellie after getting each point to get something new.