the same casual mind-set
The debate over uniforms in public schools encompasses many larger issues than simply what children should wear to school. It touches on issues of school improvement, freedom of expression and the “culture wars.” It’s no wonder the debate rages on.

A new trend is the mounting pressure to establish dress codes for teachers. Apparently the same casual mind-set toward revealing outfits is cropping up in the ranks of our teachers.

Regarding the part of the same casual mind-set, what does "casual" mean exactly?
Of course, the word "casual" has various meanings, but it seems like to have some negative connotation here.
If possible, could you give me some synonyms for this "Casual" Thank you very much :)

Question

the same casual mind-set
The debate over uniforms in public schools encompasses many larger issues than simply what children should wear to school. It touches on issues of school improvement, freedom of expression and the “culture wars.” It’s no wonder the debate rages on.

A new trend is the mounting pressure to establish dress codes for teachers. Apparently the same casual mind-set toward revealing outfits is cropping up in the ranks of our teachers.

Regarding the part of the same casual mind-set, what does "casual" mean exactly?
Of course, the word "casual" has various meanings, but it seems like to have some negative connotation here. 
If possible, could you give me some synonyms for this "Casual" Thank you very much :)

Answer

Yes . This meaning has a negative connotation
1) the same unconcerned mindset toward revealing outfits is cropping up in the ranks of our teachers.
2the same relaxed attitude toward revealing outfits is cropping up in the ranks of our teachers.
3) the same disinterested  mindset toward revealing outfits is cropping up in the ranks of our teachers.
 > When I told him that he won the the lottery , his reply was too casual to believe!

Answer

Casual = informal, when we are talking about clothing.  Casual dress means you don't need to wear a tie and suit.  Casual can also mean "not a big deal" or "don't care about ".  In this article the meaning seems to be using the meaning of "don't care about".  It seems to be saying that the teachers do not have a problem with wearing revealing clothes.  The teachers are not worrying about whether their clothing is revealing.