In The Cursed Child, we see that Harry and Co. use Floo powder to come to Hogwarts and talk to McGonagall about Albus and Scorpius. This means that Floo Powder can be used to come in and out of Hogwarts. Even if The Cursed Child is not counted as one of the 'mainstream' books, in OOTP, we see that Sirius is able to use Floo Powder to talk to Harry.
From the above lines, it is quite evident that students may be able to use Floo Powder to come to, or get out of Hogwarts. Is there any mention in the books of students using Floo Powder to come to school?