DD started her period in the summer between Y5 and Y6 so she had a bit of time to get into the swing of things. In Yr 6 she had a little zip up pouch in her school bag that always had a couple of different pads, a spare pair of underwear, a little waterproof bag for any soiled underwear (I got it with some reusable pads I bought years ago) and a little travel pack of wet wipes in case she needed to clean/freshen up.
She is in secondary school now and is pretty well into the swing of things so she just keeps a smaller pouch in her blazer with a couple of pads/tampons and a little pack of wet wipes when she is on her period. She also has a locker to leave an Emergency Kit in so ai have made her a slightly more general kit with anything she might need that all fits in a makeup bag. As well as pads/tampons, spare pants in a ziplock bag she also has spare tights, mints and dental floss, a little roll on deodorant and a couple of those small perfume samples, spare hair ties and bobby pins, power bank for her phone, a handful of various sized plasters, wet wipes, paracetamol, hand sanitiser and hand cream, lip balm, stain pen bc she is always spilling something on her white shirts, safety pins in case she loses a button midway through the day, tiny screwdriver kit and screws for her glasses, a basic stationery set that you can usually get from the pound shop - pen, pencil, ruler, rubber, sharpener - to get her through the day if she forgot her pencil case etc.
It was invaluable as at least every item was used once at some point in the school year if not by DD then by a friend who didn't have something and asked DD who did, I think she brought it home at Christmas/Easter hols so we could replenish a few things that were running low but otherwise it stayed in her locker 24/7.