Robione,
The school gave you the right information. For any new FAA ratings to be added to your FAA certificate, the FAA certificate must be stand alone. Since your PPL is issued on the basis of your JAA licence, it is restricted to same limitations and priviledges of JAA Licence.
Of course this is true only if you want to get stand alone FAA license. If you just want to come here and fly, you can use your current FAA certificate as long as your JAA licence and medical and FAA BFR are current.
Cheers,
DesiPilot