Yes, if you are providing security guards to an educational institution then the same is exempt from GST.
But keep in mind that, here educational institution means "institution
providing services by way of pre-school
education and education up to higher
secondary school or equivalent".
your service is taxable under RCM for the institution. even hostel provided by the institution will attract gst. exemption is only for the service " pre school...so n so as quoted above.