I'm an everyone pays for themselves with meals out person, and while I would find this odd from a school mum, it wouldn't bother me in any way, I'd just decline.
In our family parties are hosted and therefore the host pays for venue/food (we all sort our own drinks), but meals out at a restaurant are a different kettle of fish entirely, people choose to come or not come, but if they do they pay for their own families.
We've never had any issues doing things this way and my family go out a LOT together, it's actually the fairest way of doing things IMO, no expectations, no-one taking the piss, no need to reciprocate, or feel hard done by if people don't, just turn up eat and drink what you would like, pay for what you have had, have a great time then go home.
DH family have a 'person inviting has to pay' ethos, and I think we've all been out together twice in 10 years because of it, none of the siblings ever offer and they will only ever turn up if the parents are paying.. I find it really grabby and don't usually enjoy it, I also don't feel like I can order what I would like, I try and keep costs down for the payers, which I wouldn't do if I was paying for myself, I find the whole thing discombobulating.