Hello PREMARATHNA B.A.P.D,
Thanks for your question.
If you want to continue use, It doesn't renew automatically. You must renew the subscription.
When expires, you won’t be charged automatically. You can choose to upgrade to a Pay-As-You-Go subscription if you wish to continue using Azure services. This upgrade will allow you to keep using the same resources and services, but you'll be billed based on your usage.
Also please note the the subscription gives free access for select products for 12 months not 30 days. This is documented here:
See: https://azure.microsoft.com/en-us/free/students
Please let me know if you have further questions**
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