Do we need to report low platelets as a long-term health condition, or does it go away after being treated? My teen is getting ready to go to college.

Reason is important. The reason for the low platelets is important, as is the platelet count. As long as your teen is aware of the condition and what to look out for, that is ok. If they are headed to college, a medical record should probably be sent to the college health clinic.
How low & cause. Who would you report it to? Low platelet counts, as long as above 40 k, are seldom associated with signs or symptoms. It is important to determine the cause of low platelet count as that will determine the long term implications.