Yeah, I'd go with foam core too. If it comes down it's not gonna hurt anyone, it only has to last for graduation day, so it doesn't pay to over engineer it (or the cost). Just flatbed print it, or lay a print on it, It's easy to cut, don't even need to laminate it if you don't want to, easy peasy. If they want to be able to re-use it again in years to come, then go with something longer lasting like PVC or Sintra that won't get damaged as easy.