Of course, a sudden rainstorm is only one of many handy excuses to get manga characters naked. You could also make them...
- Change for PE together.
- Change into staff uniforms for part-time jobs.
- Try on costumes for the school festival.
- Spill liquids on clothes that must be cleaned immediately.
- Be forced to share a room and bath after missing the last train home.
- Or, my personal favorite: Get so drunk or delirious with fever that they require assistance donning pajamas before bed.