Is there a legal way to require employees to reimburse the company for their tuition assistance if they leave within six months or a year from receiving the training?

If the employee receives training or certification during work time or as a requirement of the job, you can't place any restrictions on when they leave the company. When training takes place during work hours or is a job requirement, you might consider a retention-incentive program such as future stock options or bonuses for staying with the company.

If you provide tuition assistance for voluntary training that's not a requirement of the job and is taken on an employee's own time, you can require them to reimburse you if they elect to leave the company before an agreed upon time. Have all employees sign an agreement acknowledging that if they receive tuition assistance and don't remain on the job for at least six months subsequent to the education or training, they must reimburse the company. If they choose to leave before that six-month period is up, you can deduct the tuition costs (or most of it) out of the employee's last paycheck.