Would you think this is fair?

Today was my last day of school, it was a non school uniform day due to the hot weather and i wore a purple top that only showed a little bit of my belly turns out the school didn't like that i was showing the littlest bit of belly but can let all the other girls in my school have shorts that go up their bum and allow that but dont allow me to show a tiny bit of belly and force me to put on a hoodie i bought with me because i was going to be out after school and it was for when it gets cold. but i refused to put the hoodie on and locked myself in the toilets. They were threatening to get the site team to bang the door down if I didn't come out. so in the end I came out and started walking around. Then I finally decided to put my hoodie on and if I took it off they would send me home but I didn't. so at the start of period 2 i went to reception and asked if i can go home and change which they let me but told me not to come back. Now for the last six hours I have been sitting in my room with no food or drinks in this hot weather. So I finally decided to go downstairs and get a drink. When I did my dad started to scream at me for showing the slightest bit of skin telling me im a slut and all this then he next said I'm grounded for a week because of it and can't see my boyfriend on his birthday. So what should I do?

This is incredibly disgusting. A bit of belly won't hurt anyone, and it's a nonuniform day. You're not dressing "inappropriately" (notice the quotation marks) every single day, and it's completely harmless for you to be showing a bit of belly. If your clothing covers your bum and cleavage completely, that should be good. Ask the school/your dad why the heck they think it's wrong for you to show skin, especially since belly skin is the EXACT same as the skin on your face.