How to SELECT this in MySQL,

Table structure
Code:
lessonid | classid |
--------------------
1   	 | 4
2   	 | 4
5   	 | 5
6   	 | 5
7   	 | 7
11  	 | 7	 
12  	 | 7
From the table structure above, How to select classid's that has the lowest number of lessonid only?
So the result above should look like this below,
Code:
lessonid | classid |
--------------------
1   	 | 4
5   	 | 5
7   	 | 7
By the way lessonid is the primary key.

Thanks in advance.