Recent studies have shown a significant association between high CO2 levels and impaired cognitive performance.
http://dx.doi.org/10.1289/ehp.1510037
The study has potential implications regarding the indoor environment in which ACT/SAT tests are given, particularly the levels of CO2 from respiration and fresh air ventilation rates.
Arrive early and sit close to the fresh air vents!
I’m sorry, but I think that sitting by the vent would only be good for a breeze and that thr difference in CO2 levels would be negligible and may be bad if you are too cold.
…or you can take the test in a warmer climate, in classrooms with direct access to fresh air, ideally ocean air. which is high in negative ions. Ask your proctor to open the windows and/or the door to let the fresher outside air in.