Our class teachers always have to teach the 1st lesson

Please, see this article first: How can I specify class main teacher?

If this class teacher teaches only one subject, you can simply lock their lesson on that position. See these articles:
Moving the cards manually
Locked cards

However if the teacher teaches more subjects or they have double and single lessons, then it is better not to lock these cards, but rather specify it in the Class - Constraints.

Note: This option consider always only one period per day. So, even in case, you select two periods (e.g. 1st and 2nd) the constraint will be considered as fulfilled, if the class teacher will have only 2nd period in this class.