To set Student Admission Number automatically go to System Settings > General Setting >Student Admission No. Auto Generation and change Admission No from Disabled to Enable.
 
To set values for Student Admission No. Auto Generation here it is recommend to set prefix any alphabet character like SA and if you select Admission No Digit as 4 so you must set Admission Start From value of 4 digit like 1000 or 1001 otherwise it will show error. Also make sure auto generated admission no. will not conflict in future with any existing admission no. (with manually entered Admission No. value).