not received gratuity from school

My grandmother worked worked for a school as a teacher from 1969 to 1994 for about 25 years. She is eligible under the amendment of payment of Gratuity Act 1972. She is now 80 yrs old and her collegues have got their dues but the school has harassed her and said she is not eligible. Can somebody tell me if she is eligible and what process to carry out to give her justice? Is there a specific time within which she should have applied for her gratuity dues?