All Students at all accredited higher education institutions in US are eligible for DreamSpark at https://www.dreamspark.com. There is no specific work necessary on the school's part to 'enroll'.
Given that there is No single method of verifying a student status across all higher-ed institutions in the US, there are a number of mechanism that DreamSpark uses:
Even though not all schools are listed on the DreamSpark site, all higher-ed students from all US schools use the mechanisms 2,3,4 listed above to get their DreamSpark software.
On the DreamSpark site (at https://www.dreamspark.com/wayf/wayf.aspx) just below the list of schools is the following -- click on the “Learn More” button to get verified through JourneyEd http://www.journeyed.com/MSTools.
So, No your school does not have to be listed on the DreamSpark site for a higher-ed student in US to get the DreamSpark software just click on the “Learn More” button it to get verified as a student.