These are special prices offered to members, when they go on general sale in June they'll be higher and I assume the match-day prices will be higher as well, probably £30+. I'm considering a S/T but I know there are quite a few games I can't make which is why it's years since I last held a S/T...
Looks like the lower school end is for us all next season (I assume from the Twitter-picture that season tickets are available there!).