Under normal circumstances this would fall under weapon on school grounds if it's in his vehicle. However, as he's part of the school rifle team (didn't know there were any left!) the school can (and should) give him permission to have the rifle locked in his car on school grounds.

He could check with the rifle team coach, then bump it up to the Principal or Superintendant's office. If they sanction it then he should be OK. The statute (below) is very specific about school sanctioned events be OK. If he does it without their permission and somehow the rifle is found while the vehicle is on school property it will turn into a nightmare.

Sec. 53a-217b. Possession of a weapon on school grounds: Class D felony. (a) A person is guilty of possession of a weapon on school grounds when, knowing that such person is not licensed or privileged to do so, such person possesses a firearm or deadly weapon, as defined in section 53a-3, (1) in or on the real property comprising a public or private elementary or secondary school, or (2) at a school-sponsored activity as defined in subsection (h) of section 10-233a.

(b) The provisions of subsection (a) of this section shall not apply to the otherwise lawful possession of a firearm (1) by a person for use in a program approved by school officials in or on such school property or at such school-sponsored activity, (2) by a person in accordance with an agreement entered into between school officials and such person or such person's employer, (3) by a peace officer, as defined in subdivision (9) of section 53a-3, while engaged in the performance of such peace officer's official duties, or (4) by a person while traversing such school property for the purpose of gaining access to public or private lands open to hunting or for other lawful purposes, provided such firearm is not loaded and the entry on such school property is permitted by the local or regional board of education.

(c) Possession of a weapon on school grounds is a class D felony.

Now....that takes care of the legal part for possession. Personally I would be very careful about leaving it locked in the car in the school lot. Maybe the school has a secure area where it can be locked up during the school day inside the building. Again, my suggestion is check with the school authorities to see how they want to handle it. If they have a rifle team then you son is not the only one in these circumstances.