I am not sure we even have to debate what is sexual or not here.
First an adult is in a power imbalance with a child, so no relationship other than as a friend, family member or professional can be justified, otherwise, as others have implied, there is a betrayal of trust.
Second, any touching or grabbing in a non-consensual context is assault and inappropriate whether it involves a part of the body thought to be 'sexual' or not.
Therefore this debate has absolutely nothing to do with the subject under consideration here, ie as to whether breasts are 'sexual' or not. This is not wrestling, this is touching inappropriately, and is wrong.
This also has nothing to do with the accidental (office) touching being debated in another thread. I suppose it is just conceivable that one wrestle could result in an accidental touch, but what is described here is a pattern.
Finally it should not matter whether a woman is wearing a bra, is brafree or is top free, if someone touches her breasts, accidentally or otherweise. That is not the issue.
If it makes anyone feel better I did once mistake a woman's breast for a door handle - we both survived the incident.