My plan:
1. do a C (or Rails or Node) app, that will run at server and listen to a port
2. the RFID reader will be set to send to this server/port
3. this app will trigger some SMS provider (by calling an url) to send a message
4. this app will also write some table (postgresql, mysql, or another database server) to log this entry, and set if the user are inside or outside
5. this app will also write in some fedena table (or call some function) to set if the student its in or out
6. a script will be created (to be placed in cron) to scan the table, check for absents and send sms
I don't know the structure but I think that this project will be something like that.