she's so young and you've labelled her already, in a horrible way.
Try and communicate with her in a positive way, and give her controlled choices. Pretty much what Mdamedubain has suggested. if she chooses to keep her PJs on, then the consequence is that she goes to school in the PJs.
The trick is too keep your tone light and keep the situation moving forward. Look pleased (but nnot too OTT)
'wow, you're out of bed already?! Great! Now, are you getting dressed before or after breakfast?'
DD - 'I'm not getting dressed!'
mum - 'your choice. Myself? I wouldn't fancy going out into the cold in my PJs'
Then walk away.... don't stay and battle. DO let her know that you're leaving in 10 mins... 'are you wearing PJs or school clothes?'
If you do get to the point where she's not dressed on time, don't start yelling that you're going to be late. The consequence for bweing late for school, is being kept in at break to complete work (?? or other consequence).
When she does comply, don't go OTT and say how proud you are of her. A friendly hug and something along the lines of ... 'awesome, you're ready. Now we can have fun on the way to school.'
i would never, ever force her into clothes.
Are you getting up early enough? To leave at 8:25am, we get up at 7am prompt. Allows plenty time for DS1 to dress himslef and have a little play. Young children just don't "get" the need to rush, and your DD may need a little dawdling time in the morning!
DO ask her why she doesn't like egtting dressed, and don't dismiss her feelings. She may associate being dressed with being separated from you