Hi we are running a free student service that summarizes research papers to help teach students how to read them. Is this "fair use"?

As a general matter, this is precisely the kind of use that is generally considered to be "fair." While no single factor is ever dispositive of the issue of whether a particular use is "fair," in your case you will generally be doing very limited copying (as opposed to summarization), and you will be using the papers in a way that does not deprive the original authors of any meaningful economic benefits (indeed, you will derive no such benefit yourself). While such a use should be "fair," there is no harm in asking permission. Such permissions are generally granted gratis and are an absolute defense to a claim of copyright infringement.