How to rate student loans' credits

Moody downgraded UK student loans. The process used to rate student loans must be different from the process used to rate corporate bonds. I think that the credit rating of student loans has a greater dependence on the economy. Once the economy is strong, graduated students are more likely to have jobs and more likely to receive incomes that meet the minimum requirement for collecting back loans from students. In the UK, it is easy for the government or the loan collecting agencies to collect loans from students, as the payback is directly drawn from the system like income taxes and graduated students do not have the choice to default their loans. This gives the loans more security. This is made possible under the government management; if there is no central government intervention in student loans, the risk of defaulting loans increases. Therefore, the student loans in the UK should be less risky than those in other countries which do not central government coordination.
However, the central government can increase the risk of student loans, as the central government can change the policy in order to change the returns of the student loans. For example, the central government can increase the categories belonging to the disadvantaged group that is offered some cut in their student loan payments. The central government could also increase the minimum requirement for collecting loans or change the tuition fee instantly, these two policy changes will affect the expected return from student loans. Therefore, the credit of student loans could be different under different governments. In the UK, once there is a Conservative government, the expected return from student loans should be higher, as the Conservative government is more likely to increase the tuition fees; while a Labour government is more likely to cut the tuition fees and lift the minimum wage requirement in order to collect loans.
Therefore, the credit rating of student loans is not necessary to depend on students' individual characteristics, but it depends more on the exogenous factors, including the economy condition, the labour market situation and etc.