I want to tell a woman that her bikini is to revealing.
That seems dangerous. Why do you want to tell her that her bikini is too revealing?
Because she needs to know that her bikini isn't appropriate for the occasion.
It can be annoying when someone wears clothing that isn't appropriate for an occasion, but it's not appropriate for you to dictate what a woman wears. If you tell a woman not to wear something, odds are you will only just offend her rather than change her behavior. Let her observe the attire of other people and make a decision herself about what is appropriate.
She needs to be told that it's not appropriate. In this case, it's a school trip and she should have dressed more conservatively if she is going to be around the students.
But is is not your responsibility to do so. Commenting on what other people are wearing can get you into trouble. They might think you are a pervert for paying so close attention to what they are wearing.