I am working on a school project for which i need to use the Arduino GSM Shield. However, i have been checking for the past couple of weeks and it has been sold out. Would someone have any idea whether the item has been discontinued? if not, how long does it usually take to restock?

You can get one from:

Sparkfun - SparkFun Cellular Shield - MG2639 - CEL-13120 - SparkFun Electronics

Adafruit - Adafruit FONA 808 Shield - Mini Cellular GSM + GPS for Arduino : ID 2636 : $49.95 : Adafruit Industries, Unique & fun DIY electronics and kits

Seeedstudio - Seeed Studio Bazaar, The IoT Hardware enabler.

All these companies have one or more variations of some sort of GSM cellular shield or breakout. Just search "GSM" on their sites. The examples I gave are not the only GSM related product they offer.