Sure abstinence = no pregnancy = no STI.
But we also know that abstinence
education, which is what we are talking about here rather than abstinence as a behaviour, does not correlate with reduced sexual activity. In fact, the research tells us (snippet of one research article posted below) that abstinence only education is correlated with
higher pregnancy rates. So to be blunt, the school if it emphasized abstinence only education, actually increased the risk that its student body would a) engage in unprotected sex, and b) experience teen pregnancy. So one could argue the school shares some of the responsibility for the situation.
"Among the 48 states in this analysis (all U.S. states except North Dakota and Wyoming), 21 states stressed abstinence-only education in their 2005 state laws and/or policies (level 3), 7 states emphasized abstinence education (level 2), 11 states covered abstinence in the context of comprehensive sex education (level 1), and 9 states did not mention abstinence (level 0) in their state laws or policies (
Figure 1). In 2005, level 0 states had an average (± standard error) teen pregnancy rate of 58.78 (±4.96), level 1 states averaged 56.36 (±3.94), level 2 states averaged 61.86 (±3.93), and level 3 states averaged 73.24 (±2.58) teen pregnancies per 1000 girls aged 14–19 (
Table 3). The level of abstinence education (no provision, covered, promoted, stressed) was positively correlated with both teen pregnancy (Spearman's
rho = 0.510, p = 0.001) and teen birth (
rho = 0.605, p<0.001) rates (
Table 4), indicating that abstinence education in the U.S. does not cause abstinence behavior. To the contrary, teens in states that prescribe more abstinence education are actually more likely to become pregnant (
Figure 2)."