You absolutely can be compelled to incriminate others. If incriminating others actually would incriminate yourself, any protection that gives you can be nullified by giving you immunity. Generally, if you refuse to testify when compelled, outside of few circumstances (self-incrimination, various privileges) you can and will be punished. It's important to be clear about this because it's important knowledge for anyone who has to live under the legal system.

Of course the other kid would have been expelled, see the final sentence of my comment (quoted below).

This is just the result of a culture built around the highest rate of incarceration in the world, the harshest punishment is expected in every situation both by those giving it and those receiving it.

Whether you could be compelled to give up a peer in this situation in the context of a court (you absolutely could and would be) isn't relevant. It's a flawed premise to analyze this situation through the lens of American law even if that were the case, because American law is not designed to help those involved or provide the best outcome for society. This is a story about children being treated like criminals, we shouldn't be discussing it as though they're literally on trial. None if it would be a problem if not for the environment fostered by schools where students expect the harshest possible punishment rather than adults to work to help them.

If the administration were interested in helping the children, the silence wouldn't be necessary because they children would understand that a classmate with harmful intentions would be helped by the teachers, and a classmate who brought a gun for harmless reasons would simply be told calmly why that isn't acceptable. Instead, you have a country with a carceral obsession creating schools which only know how to punish.