I'm going to continue my higher education and about to leave my current jobs. I've already told my manager that I'm going to leave but I've not handed in my official notice yet. Is it a good idea to asking for a raise before quitting to improve negotiability in the future job search.
If you've already put in your resignation, there's no point in asking for a raise. Your employer has absolutely zero reason to give it to you, and it will only make you look bad. I cannot imagine a scenario in which you'd be successful.
As to your worry about being able to negotiate your future salary, your past salaries should not affect how much you ask for in the future. Negotiate based on the market and what you think you deserve, not what you have been paid in the past. In some places it's even illegal for interviewing companies to ask for your previous salary. Besides, if you are going back to school, your value should be increasing after you've received more education, so your current salary should not matter.
If you want to know how much salary you should be asking for in the future, I recommend taking a look at this other question: How can I determine a reasonable salary to ask for?
You'd just about kill any chance for getting a good reference.
Doing so would be considered the nadir of unprofessional behavior. Some companies take it very personally when you leave. I know one company where, if you leave, you may never be rehired.
So, if they are already a bit unhappy that you are leaving, and then compound it by asking for a raise on your way out the door, it's highly likely that they would consider it a slap in the face.
Look at it from the employer's point of view. You just cost them money by announcing that you are leaving. You're going to add insult to injury by asking to cost them more money without any benefit to them. How well do you think this would be received?
Yes, you can, and I recommend you do. But you're unlikely to get the raise - because the company gains nothing by it.
If you were to say "i'm going to company X unless you give me a raise" that's fine, it makes sense. But unless you can believably say "i'm going to university Y unless you give me a raise" then why would the company give you a raise?
In this case, you've already told your boss you're going... so they don't really have an incentive to give you more money.
I haven't had the same experience as Don - heck, I've gone to study and come back to the company I left from, I don't see why a company wouldn't want to re-hire someone they already liked and is now provably more valuable.
But it's hard to make a real case for a pay rise, but I do suggest you try it to get used to asking for more money.