Normally employment bond has no value and is illegal to sign bond. In your case, it is different. You were given 2 years education/ training in the college and after completion of training you were absorbed as Faculty Member/ scientist/ research associate, as the case may be. Here you are bound by the terms of bond to the extend of training costs incurred by the institution to train you. It is right that if you leave, the institution will have to find another person with sufficient training for which the institution has to wait for yet another two years. If we view the matter in this angle, the institution has every right of claiming bond amount or insisting you to continue for the next two or three years, as may be prescribed.
On the other hand, if the institution has not incurred any expenditure for parting education/ training to you, then the institution cannot compel you to continue or demand bond amount.