Can magnetic tablets be brought on board a plane?

This is not possible.

According to civil aviation regulations, items that passengers are prohibited from carrying or checking in include: guns, military or police equipment, and controlled knives; flammable and explosive items; poisonous items, corrosive items, and radioactive items; and other items that endanger flight safety. Items, such as strongly magnetized objects that may interfere with the normal operation of various instruments on the aircraft, items with strong pungent odors, etc.; other items that are prohibited from being carried or transported as prescribed by national laws and regulations, as well as imitations of the above items