Blinding and randomization
After baseline assessments (January 2019) schools were randomized (by a researcher not involved in the current study using a computer-based, random number generator) with an allocation ratio of 1:1 to either intervention or waitlist-control conditions. School allocation to the intervention or control group (i.e., usual care) was blinded to all data collectors (aside from the project manager responsible for correct delivery of intervention materials). Schools allocated to the wait-list control group received the PLPL program in full during the following academic year (Fall 2019), but did not contribute data to the trial when receiving the PLPL program. Notification of group allocation to either group was completed by the project manager via email.