Hi @AnjalyMore,
SIS is an acronym for Student Information System. In most education areas, the student information system is considered to be the source of truth for student, teacher, enrollment, course, grade, and other information. Many of these systems have functionality for teachers to send messages to students of courses they teach, but there are some systems which may not allow this. Some examples of student information systems are PowerSchool, PeopleSoft, and Banner, though there are many many more out there. You'd want to ask someone in the IT area of your school/institution about what SIS is in-use and whether it has any messaging/email abilities or not.
Hope this helps!
-Chris