for a school trip and driving her instead? The coach only has lap belts and the school don't want her to use a booster seat.
She is the size/weight of an average 3 year old. In my opinion, if there is an accident she could be seriously injured wearing a lap belt around her abdomen as the belt is provided for adults not small children.
I'm sure they think I'm being overprotective but I've read the advice on car seats and there's a reason why small children are not supposed use belts around their abdomen.