Complex sentence: From the note left about it being the end of the school year, a block wouldn't be necessary at the time, leaving a warning would be fine and then revert the vandalism if it hasn't been reverted already.
Split into simpler sentences: From the note left about it being the end of the school year, a block wouldn't be necessary at the time. Instead, it'd be best to leave a warning and then revert the vandalism if it hasn't been reverted already.