I discourage direct emails or PMs when helping.
Posting the answers and questions on the forum let everybody or somebody in the future with same needs will be benefited also by just reading the posts.
For your question on the architecture? Please elaborate on this.
Hardware architecture or software?
For software, the application is straightforward and IMHO does not need even complicated charts or UML.
You just need to parse the SMS received for the student ID. Using a standard format for the SMS query helps a lot.
For example: GRADE 12345. You need just to trim spaces at the beginning of the text received and copy the ID number at the 7th position. You need also to compare the string "GRADE" if present else just ignore the text.
After you have the ID number, just do a lookup on your grades database and send the grade back.
You also need to retrieve the sender's number to be able to send it back.
As suggest by gigz, you can use a GSM modem. If you follow this route, you need to familiarize and do research on AT commands. http://www.developershome.com/sms/atCommandsIntro.asp
Serial communications knowhow is also needed.http://www.google.com.ph/search?hl=tl&rlz=1C1GGLS_enPH323PH323&q=serial+communications&btnG=Maghanap&meta=&aq=f&oq=
For the hardware, a datacable is just needed and connected like this:
CELLPHONE <---------------------> COM1 / COMPUTER
GSM modem <--------------------->COM1 / COMPUTER