Don't want to use my supervisor as a reference so not sure what to do

I'm currently in the process of applying for nursing school and I'm having a difficult time with the reference. Luckily the program I'm applying for is only asking for one reference, however, they want one from a Supervisor or professor. It specifies no co-workers, clergy, friends, family, etc.

My issue is I graduated from college 10 years ago so all of those relationships are gone. I have gone back and taken pre-requisites but those have all been completed online.

I am enrolling in another evening class that will be in person this next semester to hopefully alleviate this problem in case this doesn't work out so I can have a current academic reference.

However, the deadline for this reference is that it must be submitted and received no later than February 28th so that wont really work for the time frame.

I have been in my current job for 4 years. I do a good job and was designated as a mentor specifically to train new people so I know my supervisor has confidence in me. However, the turnover rate at my job is very high. When I first started it was 18 months and currently it is less than 12 months. I am very concerned about asking my supervisor for a reference. I am concerned that there is a possibility that potentially she would not provide me a good reference in an attempt to prevent me from leaving (Her train of thought being - If I don't go leave to go to nursing school where will I go?). I've seen this happen to other people in the agency I work for and currently they are in desperate need of experienced workers.

But even if she was willing to supply me with a positive reference I fear the consequences of how I would be treated until I would leave to start school (which if I started in August I would be there until at least early July) because I've also witnessed others have a hard time once people know they will be leaving.

My job requires a lot of involvement with the District Attorney's Office and Law Enforcement and we work in conjunction on a lot of cases together. I know that several of the officers I have worked with regularly and attorneys I have worked with could speak about my work ethic, etc. I also know a couple that would be willing to write me the letter and be discrete about it. Do you think that would potentially be an acceptable substitute?

My prior work experience has been abroad and contacting those former employers is not really an option as one I can't reach and the other two have retired and I don't know where to locate them.

Any other suggestions?