This is what worked for me, started on about 18K 3 years ago upgraded the schools infrastructure helped them developed their MIS Systems, Electronic Registration, Reporting\Assessment sysetems. Then just explained the situation to Management that if I was to break my leg or have an accident or moved job as I was the only one who Managed and understood these systems they have issues. Extra Pay suddenly appeared and also another Technician to help out now on just over 28K.
I think the answer is to try to make yourself as Indispensible as possible, then if you don't feel you are getting what you are worth move.