Why didn't Professor McGonagall scold Parvati and Lavender?

Question

When Harry and Ron are having a sword fight with fake wands at the end of Professor McGonagall's class, she told them off.

“Potter! Weasley! Will you pay attention?“

Professor McGonagall’s irritated voice cracked like a whip through the Transfiguration class on Thursday, and Harry and Ron both jumped and looked up.

It was the end of the lesson; they had finished their work; the guinea fowl they had been changing into guinea pigs had been shut away in a large cage on Professor McGonagall’s desk (Neville’s still had feathers); they had copied down their homework from the blackboard (“Describe, with examples, the ways in which Transform- ing Spells must be adapted when performing Cross-Species Switches”). The bell was due to ring at any moment, and Harry and Ron, who had been having a sword fight with a couple of Fred and George’s fake wands at the back of the class, looked up, Ron holding a tin parrot and Harry, a rubber haddock.

“Now that Potter and Weasley have been kind enough to act their age,“ said Professor McGonagall, with an angry look at the pair of them as the head of Harry’s haddock drooped and fell silently to the floor — Ron’s parrot’s beak had severed it moments before — “I have something to say to you all.

_{(Chapter 22: The Unexpected Task, The Goblet of Fire)}

However, immediately after, Parvati and Lavender giggle at something about the Yule Ball, and she ignores them. Harry notes that this seems quite unfair.

“The Yule Ball is approaching — a traditional part of the Tri- wizard Tournament and an opportunity for us to socialize with our foreign guests. Now, the ball will be open only to fourth years and above — although you may invite a younger student if you wish —”

Lavender Brown let out a shrill giggle. Parvati Patil nudged her hard in the ribs, her face working furiously as she too fought not to giggle. They both looked around at Harry. Professor McGonagall ignored them, which Harry thought was distinctly unfair, as she had just told off him and Ron.

_{(Chapter 22: The Unexpected Task, The Goblet of Fire)}

Professor McGonagall is normally quite fair, and I can't understand why she shows this partiality.

Why didn't Professor McGonagall scold Parvati and Lavender?

Answer 1

The answer to your question is already in your quoted text: In general teachers do not like pupils to fool around: They want them to pay attention.

Harry and Ron obviously did not pay attention: They fooled around with fake wands and fought each other.

Lavender and Parvati giggle, that is true. But they do so because of something that McGonagall just said: So they were clearly listening to McGonagall.

Let's make up a more comparable action to make it clearer: Let's assume the following:

Harry laughs out loud because of a joke that Ron told him.
Parvati laughs out loud because of a joke that McGonagall told to the class.

Whom would McGonagall tell off?

The answer to this question is most probably the answer to your question...