This 2011 NYT article discusses a number of different SMS based sex education services throughout the US. One interesting point is that SMS services (and internet services more generally) do not necessarily have to comply with the sex ed laws in a given state.
In addition to reaching students, SMS services can also reach kids who have dropped out of school. The services can also be valuable to teachers who haven't received the proper training or are legally barred from teaching scientifically accurate information in their districts.
Challenges to these programs include abstinence only education advocates and the difficulty of directing students to these programs when searching for information online, rather than porn sites or sites with scientifically inaccurate information. A successful solution for outreach turned out to be student led SMS chains.
This article examines the success of The Hookup, one of the SMS services mentioned in the NYT article. It also considers the difficulty of determining the efficacy of SMS based education services.