Yes. Students can wear a casual dress. But there are more advantages of wearing your school uniform than a casual dress.

          Firstly, school uniform doesn't look different and in the examination hall, the examiner will also not keep a special eye on you. 

       Secondly, if the students are in a proper school uniform, it definitely shows the identity of school and makes oneself and the teachers proud.

             So school uniform is always better, but it's the ID card that matters, not some uniform or dress. So, wear something comfortable on your exam day. That's the most important thing.

Hii

It is not compulsory to wear school dress in board exams but it would be better for students to go with proper attire whether it is in regular class or exam days.the first thing is what are the benefits to sit in an examination hall without uniform,it cant help you to write paper infact it might get you notice easily as you looks different from others and examiner will be able to find even your little movements and always keep a special eye on you.And you would personally feel yourself quite weird consequently it will effect your performance. School dress is not that tight which make you feel uncomfortable.
On the other side,if students are in proper dress which definitely shows the identity of school and make you feel proud and your teachers.moreover,it helps the staff and teacher to find out their school’s students easily in other school centres. Lastly BEFORE the exam day you dont need to waste time to think about the clothes which you're going to wear so simple wear uniform.

Thanks!