For navy - my son went and got the green ID card at thanksgiving - from paperwork done in august then processed at unit and he was instructed to get it during thanksgiving.
At least at VMI, the person in charge of this relayed to me that it was important that my son NOT surrender his dependent ID - that the green one issued would be reserve and only activitated while on active duty - therefore the rest of the year and certainly during the school year my son would continue to be covered (medical) by his dependent ID.
At least in the old days, those reserve ID cards need to be accompanied by orders - instructions to and fore active duty timeframe. Thus they would only be valid - useable at PX, commissary, etc during those periods. Interesting tidbit - VMI does a spring field training exercise for a few days- the NROTC (both navy and marine) midn went to norfolk and while there they were taken to PX - and my son did buy something - he only had his reserve ID with him - so maybe during the FTX days they were treated as tho they were on active duty.
Therefore I dont think one could rely on this reservist ID for hops - esp to and from school - just my opinion - todays military may certainly have different rules
Just thought I would offer what I knew or have experienced
larrrys mom