Word Usage – ‘High School Student’ vs. ‘High-School Student’: Correct Usage Explained

I believe that in this context, the two words are completely interchangeable with no change in meaning or register.

I can't actually think of a context where one would be preferred over the other, but I'm willing to be proven wrong on that.

At risk is a prepositional phrase, in which risk is a figurative location. You are at risk when you are in a continual state of risking some sort of harm. (English likes to conceptualize abstract states as locations, each with its obligatory preposition: in danger, in love, on cloud nine, under the weather, ... please suggest more examples in the comments.)

Adding a hyphen, at-risk, transforms the phrase into an adjective, meaning "the modified noun has the property of being at risk". You can use hyphens this way with any prepositional phrase. For instance, suppose you have

Jane McFamousPerson's fans in Los Angeles were excited to attend her public Q&A session last Thursday ...

you can turn that into

Jane McFamousPerson's in-Los-Angeles fans were excited ...

with basically the same meaning. However, it is unusual to do this with "ordinary" prepositional phrases. At-risk gets used this way a lot because it's neutral -- it expresses no judgment on how the group at risk got to be that way. That's important when you're an NGO trying to help people. So it is preferred to more colorful terms (e.g. impoverished) which, in English, often carry a weight of moral disapproval.