As the saying goes, one cannot clap with one hand.

I'm not willing to believe that Harriet isn't looking forward to joining the class that Emma is showing her.

She *has* a conscience!
"I do not think he is conceited either, in general," said Harriet, her conscience opposing such censure; "at least he is very good natured, and I shall always feel much obliged to him, and have a great regard for -- but that is quite a different thing from -- and you know, though he may like me, it does not follow that I should -- and certainly I must confess that since my visiting here I have seen people -- and if one comes to compare them, person and manners, there is no comparison at all, one is so very handsome and agreeable. However, I do really think Mr. Martin a very amiable young man, and have a great opinion of him; and his being so much attached to me -- and his writing such a letter

She's able to give her opinion and oppose Emma! So what's stopping her?

She's able to judge and weigh Emma's words here in this conversation...even contradict;
CH:10
"Dear me! it is so odd to hear a woman talk so!"
(a very ordinary normal Harriet here)
[...]

"Ah! so you say; but I cannot believe it."
(contradicting)

"But then, to be an old maid at last, like Miss Bates!"
(contradicting again)

"But still, you will be an old maid -- and that's so dreadful!"
(Not only contradicting again, but not accepting)

"Dear me! but what shall you do? how shall you employ yourself when you grow old?"
( shows some doubts about Emma's ability...and is not even accepting the class/money thing that Emma is offering as something superior that will come to her support in old age)