This is technically true, and your health teacher probably is only looking for trued, but there is more too this. Abstinence is the only way that is 100% guaranteed. Condoms, however, have a 99% success rate and that combined with getting a free STI checks from your local health clinic every few months and asking your partner about their status before having sex, will keep you from getting an STI unless there is some sort of freak accident. No one is going to be abstinent forever, if you're going to have sex use a condom. If you are having non-penatrative sex, you should use a dental dam to avoid STI's. You can make a dental dam out of a condom by unrolling it a little bit, taking a pare of scissors and carefully cutting off the tip and slicing down one side so it becomes flat, then you can unroll it the rest of the way.
