Students have rights, but it is well-settled that they do not have the same rights as adults. The Bill of Rights applies to schools, but cannot be applied wholesale to minors. There is an analysis where the rights are fitted in to the unique requirements of the school context. The only precedent I'm aware of in the context of the 5th amendment is J.D.B. v. North Carolina (2011), which found that a uniformed police officer who questioned a student on school grounds about a robbery should have given the student a Miranda warning. It's a huge leap from that to conclude that school administrators can't ask about drug use on a survey in a non-custodial setting.
Minors have largely the same rights as adults. There are reasonable restrictions on those rights, but the default is not "no rights with a few exceptions". It's "all rights with a few exceptions". For example, schools are allowed to place some restrictions on students' speech, but they cannot forbid speech unreasonably (e.g. cannot require pledge of allegiance; could not force students to remove black armbands protesting Vietnam War).
And no one said the school can't ask about drug use. But the students can refuse to answer.
