Question

The students accompanied by their teacher ……(has, have) gone on a picnic.

Answer 1

have.......................................

Answer 2

The students accompanied by their teacher have gone on a picnic.

'Have' is used due to Subject Verb Agreement. That is, both the subject and the verb should either be singular or plural.

Since students is in plural form, the verb should also be in plural form.

Therefore have is used which is the plural form.

Whereas, has is singular and doesn't obey the Subject Verb Agreement.