What is the obligatory Shar'i action to take towards a colleague who caused financial loss to teachers due to incorrect travel ticket bookings, knowing that he initially declared himself as a guarantor and then disavowed responsibility?

The ruling differs depending on the true nature of the colleague's work. If he received the amount to hand it over to the office on your behalf, then he is your agent and is only liable for negligence or transgression. However, if he works with the office and receives a wage, then he is an agent for the office, and his ruling is the same as the office's. You have the right to demand your due from him or from the office. This is confirmed by his request that you do not contact the office and that he is the guarantor. The agent is like the guarantor, so you may demand from whomever you wish. Disputes require hearing both parties.