Sorry for the delay!
Haru: He’d try to spend as much time as he could with them at school; instead of lunch dates, he’d make his s/o bentos (probably with mackerel) and treat every lunch period as if it were dates.
Makoto: Makoto too would be disappointed, and after some conversation with his s/o, he’d try to talk to his s/o’s parents to try to make compromise with them, hoping they’d have a change of heart.
Nagisa: The cupcake would probably complain that he wouldn’t be able to take his s/o on cutesy dates; though he’d try to work around it and try to convince his s/o to tell their parents that they were out with a couple of friends.
Rei would probably be the the chaperon of their technical date.
Rei: Rei would try to think of the most logical thing to do; in the end, he would go to the parents of his s/o to try to convince them that they were a good match for each other, and that he promised not to do anything out of bounds. If that fails, he’d probably mope a bit since he wouldn’t spend as much time with his s/o as he’d like.
Rin: Rin would be one of the one who’d try to get his s/o to tell their parents that he offered to be their tutor, when in all reality, no homework would get done. If it would work, no schoolwork would get done, it would just mainly be talking and possibly cuddling
(and some possible frick frack if they kept it super super quiet).
Nitori: He’d tell his s/o to come to his swim meets, so that way they could talk and hang out for a bit after they were finished or if he was taking a break during. He didn’t like the fact of being secretive from his s/o’s parents, but wouldn’t know what else to do, and the poor flower probably wouldn’t be able to approach his s/o’s parent/parents without becoming a nervous wreck.
Momo: He’d complain a bit; no doubt about it, though it would be the one time where he would sigh and do the right thing (probably after trying to have a secret date, but his s/o would get mad at him if he kept them too late and their parents got suspicious). He’d go to the parents and try to convince them, but if all else fails, he’d have them go to his swim-meets and he’d try to eat lunch with them at school as much as possible.
Sousuke: Sousuke would probably immediately confront his s/o about telling their parents that they were in fact a couple, and despite their rules, that he wished to date their child. If rejected by the parents, he’d just spend as much time as he could at school with them, if they were in the same class, he’d even switch seats with someone just to sit behind them.
Seijuro: Knowing that it wouldn’t be the best idea to keep their relationship closeted from his s/o’s parents for very long, he too would go to the parents to try to convince them, but if that fails, he’d think of other ways to speak to them and spend time with them.
He’d probably be the only one smart enough to actually consider Skype.