You can only arrest people (Marxist or others) if they are clearly advocating for the violent overthrow of our government (sedition). If we are in a declared war against another government and individuals are advocating the violent overthrow of our government, in favor of the enemy (treason), that is an arrestable offense.
Individuals applying for work as instructors/teachers to our youth, should have their background thoroughly reviewed and stance on our Constitutional Republic determined. If found to be anti-US, then they should just not be hired. They are welcome to their opinion, but not when it comes to our educational system.