why would it be unenforceable? it's not like you can't leave (indentured servitude). it's just that if you choose to leave before your term is up, you have to pay back the training cost pro-rata, and that sound very reasonable.
This is pretty common [edit - in the UK], and there are distinctions made in law to stop this being abused (e.g. "if you leave you owe us a million for the basic training cost")
[context, liquidated damages clauses are OK in an employment contract, penalty clauses are not]
> Case law has clarified that in order for a clause to be a liquidated damages provision rather than a penalty, it must be a genuine reflection of an employer’s pre-estimate of loss that it is likely to suffer if the employee breaches that provision. If an employer is unable to quantify the exact loss that would be suffered, a best guess of the likely loss would be satisfactory. - See more at: http://rosenblatt-law.co.uk/bulletins/are-repayment-provisio...
[snip]
> The key to a repayment sum being regarded as a genuine pre-estimate of loss appears to be that the sum specified must be compensatory and not ‘in terroreum’ or simply a deterrent to breaching the contract - See more at: http://rosenblatt-law.co.uk/bulletins/are-repayment-provisio...
http://rosenblatt-law.co.uk/bulletins/are-repayment-provisio...
My last job paid for a good portion of my masters, every time I was reimbursed for tutition there was a set amount of time I had to still be with the company or I had to repay a certain amount.
Ended up leaving before finishing my degree due to really hating the company, I was prepared to hand back 15k, but they never even bothered asking for it. Same thing happened to a coworker of mine, though it was substantially less money in that case.