Question

change into indirect speech : the principal will say ,''rules have to be followed at any cost.''

Answer 1

the principle will  be say that rules have to be followed at any cost

Answer 2

Since the reporting verb is in the present tense, the tenses of direct speech do not change.

The principal will say that rules have to be followed at any cost.

But, if the reporting verb in the question was meant to be in past tense, i.e.,

The principal said, “Rules have to be followed at any cost.”

then, the indirect/reported speech would be:

The principal said that rules had to be followed at any cost.