@Annika - Welcome to Microsoft Q&A and thanks for reaching out.
After you exhaust your available credit or reach the end of 12 months, your Azure subscription becomes disabled. Meanwhile, you cannot renew your Azure for Students Starter subscription with your student email address. To continue using Azure, you may then upgrade to Pay-As-You-Go subscription in the Azure Account portal.
For more details, please check the document: What happens after I use my $100 credit or I'm at the end of 12 months?
By the way, for more question about Azure for Students Starter subscription, you can contact here: Subscriptions Support.
Hope this helps. and please feel free to reach out if you have any further questions.
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