At 10 mnths there is no need for her to have a bottle. She will be just fine with a packed lunch and water while you are away. She doesn’t need formula. Just food and the milk you give her when you are with her. When my first child was born I was working as a teacher and went back to work full time when he was 6 weeks old. By the time he was 10mnths old I was “over” expressing, and then my health nurse told me to just stop and give him food and drinks - I hadn’t thought of it before. He went on to breastfeed just fine. You may have to express for comfort at first. It’s easy to get a private spot in a school and staff and admin will be supportive. Your breasts will soon adjust and you won’t need to express for long