The maximum penalty for first offence possession of a small amount of marijuana in Iowa is 6 months in jail. If adjudicated as a minor, you could be sent to juvenile corrections, theoretically, until you're 21.
However, a juvenile conviction has a lot less long term consequences than an adult one.
Supplying marijuana to a minor is a serious felony in Iowa, carrying a minimum 5 year mandatory prison term, with a maximum of 25 years. Therefore, one thing the courts are almost certainly going to want you to do is identify your source. If you refuse to co-operate, that could result in a judge giving you the maximum sentence he can.
I assume you have a lawyer? If not, get one NOW.