I have got a Saturday detention for missing a regular detention, but I still have to serve the first detention IS THIS FAIR ?

last Tuesday I had an after school detention for bad behavouir it lasts for 1 hour and a half hours, as I am 15 I had MUCH better things to do then sitting in detention so I didn't go. the next day my head of year told me that as I missed a detention I would be going in to a FULL Saturday detention, this last... show more last Tuesday I had an after school detention for bad behavouir it lasts for 1 hour and a half hours, as I am 15 I had MUCH better things to do then sitting in detention so I didn't go.
the next day my head of year told me that as I missed a detention I would be going in to a FULL Saturday detention, this last for 4 hours we have to turn up in FULL school uniform , sit at our desks in TOTAL silence and write a 6 page essay on why we are being punished.

MY COMPLAINT , I have just found out that tomorrow ( FRIDAY ) that I still have to serve my original detention , this lasts for 1 hour 45 minutes, my head of year said the Saturday detention was a punishment for missing the original detention, but I still MUST serve the original detention as this was a punishment for bad behavouir IS THIS FAIR ?
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