You need to ask the school people how they define that term.
Technically, people on H visas are not 'migrants' but are only 'temporary' workers.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
generally in the US the term migrant worker applies to persons that move from one job site to another job site usually following the seasons in agriculture. to be sure how that term is used on the public school application, I recommend you speak with someone in the school administration office