Teresa McKenzie, 41, wrote several letters to the pupil at Delamere Forest School between December 2007 and July 2008.
A Teaching Agency misconduct hearing was told some were addressed to "my gorgeous pirate" and "sweet pupil A".
Ms McKenzie admitted unacceptable professional conduct but offered witness statements from people including a teaching assistant at the school in mitigation.
The panel concluded she should be prohibited from teaching indefinitely and will not be allowed to apply for the Prohibition Order to be set aside until October 2014.
The panel heard Ms McKenzie joined Delamere Forest School - a residential and educational establishment for pupils with special needs - as a teacher in late February 2006 and was later appointed assistant headteacher.
The pupil, known as Pupil A, suffered from Attention Deficit/Hyperactivity Disorder (AD/HD) and Specific Scholastic Skills Disorder.
The panel heard he joined the school in October 2006, but was excluded the following year for threatening behaviour towards other pupils and staff.
In September 2007 he was allowed to return to the school and Ms KcKenzie became his key care worker and head of care at the school, developing a close relationship with him.
It was alleged that she failed to maintain professional boundaries with Pupil A, and engaged in inappropriate communications with him - both of which she admitted.
No allegations were made about the pair having a sexual relationship, as Ms McKenzie was cleared of seven charges of sexual activity with a child by a person in a position of trust in 2010.