By train to Trier and from there by bus to the Porta Nigra.
Information about public transport can be found here: www.vrt-info.de
Information on parking facilities in Trier can be found here:
https://www.trier-info.de/parken
The Antikencard offers admission in three different versions to the Roman buildings of Trier as well as further discounts around ancient Trier.
More information at: https://triershop.de/fuer-kinder/schulklassen/antikencard-trier