I have had the same problem every year of being pit section leader.
The worst was when there were two sophomores (myself included), and five freshmen.
There were two of them that respected me (both freshmen), and even then it took until the end of the season to get them to do that. One of the two ended up with me again the next year and that helped a lot.

I'm still stuck with half of them from that year, and nothing's really changed.
Some people, like your sophomore, are just jerks. He's probably either trying to get kicked out of band because he doesn't like it (you said he can't read music and has trouble focusing, two REALLY IMPORTANT things for band. Meaning he probably doesn't practice and doesn't really try) or he's just trying to get a reaction out of you.
Stay calm, try to get your director to help your section as much as possible and if needed, have your director speak to the very problematic kids.

I've found that sometimes the problem children have a stronger will than leaders do since they're trying to undermine you, and thus they kind of usurp the section if left unchecked by a higher power for too long. That may be what he's trying to do, although I can't say for sure. Also you're only one grade above him and he's only a sophomore. Almost all sophomore males are actually like three on the inside and do not respect people one rank up the high school chain. Also, his being two months older than you means nothing. He's just trying to be an a**. Try not to take it to heart.

As for the others, maybe once your director talks to the section they'll get into shape...it doesn't sound like the rest are that bad of kids, they just need discipline. Next time you catch them talking about you behind your back or disrespecting your orders, tell them that "we don't do that here." or something. That's what we do in my band and it works.

Good luck. Running a tough section is, well, tough. But it is possible; I've done it for three years. It doesn't get much easier but you do learn how to deal with it, and that's the big thing. Once you can learn what to say to the problem children to make them behave or learn when they're just trying to piss you off, most of your problems disappear.