What should I wear tomorrow?! Help!!!?

I'm 12 years old (sixth grade) and I'm used to wearing a uniform to school every day. Tomorrow we're going to a field trip and it's gonna be like 78 degrees. I have absolutely nothing to wear. I stand there,staring at my closet. I just want a casual shirt. Jeans, I have no problems with. Well,I MUST wear long sleeve--it's a rule there. Anyway,so all my shirts are just-ugh. Too long,old-fashioned,puffy,just plain UGLY! My mom just doesn't ever get me what I want. She gets me those cheap Target and Kohl's clothes which pisses me off. She refused to buy me new clothes because I 'always say I want it and then randomly say I hate it and don't wanna wear it' but that's because I have reached the age where I start to be more cautious about my appearance. She doesn't understand. So do you have any tips what to wear tomorrow? I don't even have much in my closet. Any answer helps. Any advice,tips,etc.