If I am a teacher, can my school district forbid me to attend church where my students attend?

As a general matter, an employer, whether private sector or government, has considerable latitude in putting restrictions on their employees during their employees' private time. For example, an employer could forbid dating coworkers, or socializing with customers or clients.

However, there are certain kinds of restrictions which are specifically forbidden by law. Federal anti-discrimination law, for example, forbids discriminating against employees due to their religion, and requires "reasonable accomodations" of employee religious practices--which might include not interfering with an employee's choice of house of worship (especially if there are not other churches of that denomination in the area which would be acceptable to the employer as alternatives).

Moreover, the U.S. Constitution, including the First Amendment guaranty of freedom of religion, applies against the government, which includes public schools; there are stricter limitations on what schools can do vis-a-vis religion than private employers.

Therefore, it is highly unlikely that your school district can forbid you from attending a church which is attended by students or parents from your former location, though it could  require that you do not associate with them other than the minimal amount of politeness and interaction required by attending services with them--for example, you could be expected to not be in a church club or committee (since such are not worship or religion per se) which includes people you should not associate with.