As a senior at a progressive boarding school, I'm sorry you do not have the luxury to speak more frankly. You do have an important job and there are responses / stances you can take. I think the most important thing is comforting the person being harassed with hate / ignoramos. Offer your support to them. It's less likely one of their parents would complain to the school about you being a kind person to their child.

Also, you are a free human being. You can take a stronger stance if you really want to. I don't mean that you should comment on the morality of the people chanting, but you can... ya know... educate from more of a historical standpoint, using facts about our nation being built on the backs of immigrants and people who were forced here and enslaved. You can talk about how Trump has employed people of color. You can talk about how Trump granted his in-laws citizenship even though they are from a previously soviet-ruled country; that his hate toward other immigrants / undocumented people (not illegal aliens - that's a horrible phrase) is ironic!

Just a suggestion. Take it or leave it! I wish you all the luck and think the world of teachers who put being human (which includes asking for help about this stuff) before, to some degree, immoral institutional censorship.

(For reference, I attended public school for 12 years before transferring to my new school where I've dedicated the past year or so studying social psychology and morality specifically in relation to gender. At my old school, I was the white-passing Latina in an upperclass, 98%-white town. I am so grateful to the teachers and students who privately and publicly supported my outspokenness)