in front of 是在这个物体（外部）的前面,比如说 You're in front of me.你在我前面.
你想表达的应该是这个:There are 2 blackboards in the front of the classroom.
in the front of 与前者的区别在于这是指在物体（内部）的前面.
比如说 I'm sitting in the front of the car.我坐在车的前排.
如果是 I'm sitting in front of the car.那可就要出车祸喽~我想我已经解释得够清楚了:)