I agree with you, it's hard when they are young. My daughter is 10 and she doesn't shower unless I force her to and she doesn't change her underwear unless I personally check. She never remembers to wear her deodorant so I can't imagine what it would be like if she got her period. Young girls around that age aren't always into body cleanliness. My daughter also doesn't always remember to go to the bathroom and as a result she's wet her pants a tiny bit running there when it's almost too late! My daughter also smells, and one day I noticed she wasn't washing her lady bits because I could smell them! Ew! I had to supervise her showers for a month. So I know what you're going through.
What has helped is making a schedule for her. She lays out her clothes every night and I make sure she has clean underwear laid out. She showers every other day in the morning and I do check. She has a checklist that I have to go over with her that includes deodorant.
She has to go to the bathroom after every meal, this ensures she goes at least a few times a day.
What we're trying to do is make good habits. I know it's hard getting after them about stuff but once it's a habit you won't have to. She should change her pad every time she goes to the bathroom. Please teach her how to wrap it up and buy her moist wipes to keep in her purse. Using pads are messy, and even if you change them you still feel messy all over. They make moist wipes just for this purpose and they are portable. I believe one brand of pad has a moist wipe attached to each pad. Teach your daughter to use them.
Being 11, I would imagine that they still go to the bathroom as a group in school and they probably don't have time to grab a purse with a pad it it so I can see why your daughter might not change her pad. She doesn't want anyone to wonder about her and wonder why she's bringing a pad or a purse into the bathroom. See if you can talk to her teacher and see what's up. In grade school most of the girls don't carry purses like they do in middle school so there isn't anything for your daughter to put a pad in. The teacher might need to help your daughter remember to bring her purse, or allow her to go to the bathroom alone.
Good luck!