Step 1: Click on the Communication icon on top, Choose 'SMS'.
Step 2: On SMS page by default if will be in custom SMS, enter your message and click on 'Next'.
Step 3: By default all students will be visible. If you want to select student from particular batch then use filter and select students. There is option to send SMS to students, parents and both. Once done click on 'Next'.
Step 4: Review everything and click 'Send' SMS.