Hi Muksha,
The 30 day trial is for a "production" company that you create within a tenant. If you wish to train students, I suggest using the evaluation / demonstration company in Dynamics 365 for Financials (called "CRONUS USA, Inc.") which will not expire.
The CRONUS company contains demonstration data as well, which would come in handy when you wish to train students on processes and transactions.
I hope this helps.