So if Justus was the first dean (and possible teacher),
More accurate would be "... first dean as you understand it WW"
I'm no authority you know...
the leaders/teachers might have worked something like this:
Mark ==> St. Justus ==> Pantaenus ==> Clement of Alexandria ==> Origen
And of course, it makes sense, as you said, that Justus' followers would teach what he taught. Thus, probably also what Mark taught.
Mark learned from Jesus. Mark put Justus in charge because Justus and Mark obvious had the same mind. Likely Mark did teach Justus. Maybe Justus learned directly from Jesus because he was always in the crowd when Jesus thaught. (just awild guess)
Acts 1:23, Acts 18:7 and Col 4:11 mention Justus. I didn't even read the verses (very little time now) so it could be a totally different person.
Anyway as I remebers it Mark founded the school. Put Justus in charge. Mark started to travel. Mark returned. Mark started teaching at the school. IIRC there was political/religious turmoil when Mark returned.
Maaaannn... I wish I could have met Origen. I think, in a way, I look up to him. From what I've read, he was an incredibly Godly and loving man!
Then I have bad news for you
I've heard you first gonna have a 777 year chat with the One Origen was teaching about