Some advice my dad gave me when I started HS was that when the teacher asked a question, wait and see if anyone else answers first if you already know it. You can still always answer the question a little bit late and show some knowledge without looking like that know it all kid that needs to be put on the spot by an upperclassman.
Ok, I have to jump in on this one.
If a question is asked, answer it, if your first to answer then great, this isn't high school. While ROTC is a unit it is still a competition from day one. You will compete with other cadets for positions in the battalion, placement on the battalion OML which has become increasingly important, you will compete for summer training slots. Everything is based on your standing and almost everything effects your standing.
Now I'm not saying to be arrogant or a show off, but you need to be confident, if you know the answer then answer the question. Just find a balance.
There is a funny thing about the term "That Guy", the person at the top of the list is usually considered to be "That Guy" whether they deserve the label or not.
My son got the unfortunate label of "That Guy" his first week of ROTC, he scored a 319 on his first APFT while most of the MS1's either failed or scored barely above the minimum. The talk was that he was showing off, he was aware of this and when the second APFT came along, he scored a 342. Now, he never bragged about it and offered to organize workouts and runs to help improve the class score, only a couple showed up, the three of them now have the highest scores in the Battalion.
Don't shy away from doing what you do well, be humble and help when asked, do what you can to help your fellow cadets and ask for help from those that are better in the areas you need help. Work together and you will all be better for it. There will always be some that will look at those who excel as being "That Guy", nothing much you can do about that.