How do we know when it is time to move to another job

I have been working as a school nurse for the past 2 years now. This is one job I like so much not only because the work is light but because I am in charge of the clinic, no one there to demand me on what to do every now and then. To add are the short school timings and all holidays off I get- meaning if the school is closed for half term, sem break, winter break and all breaks I am on holiday as well.These are benefits that I enjoyed for the past year. However this term our school management decided to add extra hours to our duty for whatever reason they have in mind, it bothers most of the staff as we are already used with the previous timing - which as I have said a benefit to all of us. And since we dont go together with the children they have arranged a new transporation for us. I dont mind it at first but then later on since I was not opening my opinion, they started to take advantage of my silence. I dont know if it is because of my race or they just enjoy the torture of pushing me . I was given an unfair transport dropping me to a stopped where i have to walk 2 blocks and cross a highway while the teachers and other staff gets the priviledge of being dropped exactly at their doorsteps. It really pisses me off because i have been requesting for quite some time and the issue is not resolved like they intend to leave it like that. I feel that it is really unfair since I know that i have given my entirety for the job and I have made a lot of compromise for my self just to exceeed in my performance . I know i have the passion for the children, I have good relationship with parent and make sure that they are satisfied with me taking care of their children. I am prompt with reports, organization and all. So i really feel that it is not fair to be treated this way. This has cause me to contemplate on shifting to another job. As i have been waiting for a long time for this things to be resolved yet all I get are promises that looks like bound to be broken. Little by little I am losing my motivation to pursue excellence. I started to feel tired than joyful.

So how do we know when it is time to move to another job. I cannot think now. I dont know how.