Well, I think Francois eventually gives us the update, but after understanding below issue, I've decided to modify current school system and re-issue 9 digit student IDs.
I've tried to modify /module/student.php to increase limit from 9 digits (<11) to 12 digits (<14), and also manually changed values in DB. But in the table setting for student ID, I figured that it is defined as 'Integer', which stores upto 10 digit number, while the highest digit is from -2 to +2. After all, this is why I got 9 digit student ID from my home school. They could also change it to BIG INT, which changes the value range from 4 byte to 8 byte, but the DB queries become much slower, as it gets larger.
And, in DB, it is not student id alone that I have to change the type from Integer to BIG INT. There are other associate values like food tables, enrollment tables, etc etc.
My school is in reshuffling stage, so it isn't a big deal for me. Could be an issue for others, but from a DB guy's perspective, changing data type to BIG INT for student ID will create more burden in reading data, likely resulting in slower performance of your website, and more resources for running this system.