The best placement for the sentence "This suggests that an increase in school service programs increases teen interest and participation overall." is after sentence 3, to link evidence and claim.
This placement effectively connects the statistics provided in sentence 3 (about the doubling of teen volunteering rates) to the claim presented in sentence 1 (that school involvement in community service drastically increases teen participation). By placing it here, it reinforces the idea that the data supports the assertion made at the beginning.