in the classroom 在教室裏
in the bed 睡著了，或者是躺在床上,特別是在床上呆了壹陣子了。（因為英英字典的解釋是：after a very lengthy period of time）
in bed尤指在行房過程中或者在床上做事的過程中。比如：He performs well in bed. 又如：Jack and Jane had breakfast in bed.
in the bed是指在被窩裏,
而ON THE BED就是指客觀的"在床上",比如把壹個帽子放在床上,就用這個詞組.