Circuit training. 10 stations. Set up one weight that they can all safely do. Do 30 second rotations by your whistle. Teach them to use perfect form for each station, and keep doing reps until you blow your whistle. Groups of 4 so each student is lifting for 30 seconds with a 90 second break in between sets. Make sure the 3 students who are not doing their set for each group are giving positive reinforcement to each member of the group while performing their set. Make sure they know that safety comes first and teach them how to properly spot for safety. That's a total of 2 minutes per station for 10 station, or 20 minutes for the full circuit. Go through the full circuit twice.
This is what I did when teaching weight training to middle school students. It will work great for high school freshman as well. Let them free lift if there is any time left over in the period.