You don’t say exactly how old your daughter is. But even if she’s 13, she’s now a young adult. IMHO the most important lesson for parents of teenagers is to change their approach and tone, so they no longer treat or speak to their DC like a child.
Think how you’d like to be spoken to by a family member or good friend if you’d gained a lot of weight? Would you want them to mention it to you, even out of concern? If so, how would you like them to broach it? There’s no easy way to remark on someone’s weight gain without making them feel conscious of it and possibly ashamed. Lots of young women develop unhealthy relationships with food and eating disorders following puberty. It’s so important to be sensitive and not make her feel worse.
I agree it’s a good idea to rule out a medical problem. But you need to be so delicate about it. If her weight gain is due to binge eating, she could feel very ashamed if you ‘take’ her to the GP asking for medical explanations. I’d be led by her on this and how worried she seems about the weight. Say things like: ‘If you’re worried you’ve gained weight, perhaps you should go to the doctor in case it’s because of your operation. Would you like me to make an appointment?’.
You say she’s ’not that kind of girl’. But who’s her biggest influence? The people she spends most time with. It can be hard enough for young women to deal with peer pressure and influence at school and online, even when living at home with their family to counterbalance what they’re hearing. If they’re not living at home, they’re only hearing the attitudes of the other young women at the school. Even if they’re not judging, teasing or bullying her, their attitudes may be rubbing off. If they’re obsessed by weight and eating (as many young women are), it may have made her obsessed and triggered binge eating.
This may be why the school is taking the approach it is, as it may be best practice not to remark on young women’s weight. I wouldn’t dwell on it either. I’d just keep focussing on all the positives about your daughter, showing her lots of love and keep instilling healthy eating and exercise at home. Young women’s weight can fluctuate a lot in their teenage years. It used to be called (awful term) ‘puppy fat’. They often grow out of it.
And don’t expect her to confide in you if she’s unhappy, being bullied or even having sex. Most teenagers don’t share too much with their parents. It’s hard as a parent that they have their own life. But you just have to keep being there in a loving, non-judgemental way and provide lots of opportunities for them to talk if they want to.