I'll answer the first part to get you started.
On an intrapersonal level, there is a psychological need for teens to want friendship and romance. The need for romance is one factor in the rate of teen pregnancies. Also, teens may feel stifled or trapped, so they may rebel in the form of breaking rules and that could lead to pregnancy. In terms of interpersonal factors, this could include things like peer pressure. More specifically, if a person has all their friends going to a certain party, they may be more likely to attend as well. Peer pressure is often the cause of decisions such as drinking and drugs.
Institutional factors could perhaps include things like schools not adequately educating students about safe practices. This may involve public policy as well considering that schools are required by law to follow whatever is set in the state capital or in Washington DC. Furthermore, the idea of rebelling could be tied to an environment such as a strict school. In other words, if a teacher sets way too many rules that a student deems fair, then they could be likely to break some or all of those rules. This applies at a community level as well, though maybe it's not as apparent. Another community factor could be influences such as music, tv and movies that influences thoughts/behaviors. Of course people have free will, but those media do play a role in shaping mindsets.
