Ron and Neville waoke up at the same time., Ron and Neville woke up at the same time,

Since your text is in the past tense, you need "woke up."

Bbut something was wrong. but something was wrong.

A connector like "but" can be used at the start of a sentence for emphasis, but here it sounds off because of their short lengths: it would better to connect them with a comma.

They had switched bodies because Neville had fumbled a spell last night. They had switched bodies because Neville had fumbled a spell last night.

You need to connect these sentences to communicate causation (X because Y, or X caused Y.) You can use "because" alone in dependent clauses (for example, "Because Neville had fumbled a spell last night, they had switched bodies,") but since these are two independent clauses that it connects, you can't separate them.

Because Neville had fumbled a spell last night.

They need help, but first, classes were starting in 10 minutes.

iIf they missed a whole day, teachers would notice, especiallyand so would Hermione. If they missed a whole day, teachers would notice, and so would Hermione.

Using "especially" here makes it sound like Hermione is one of the teachers

They needed help, but first, class was going to start in 10 minutes: if they missed a whole day, teachers would notice, and Hermione especially Hermionewould, too. They needed help, but first, class was going to start in 10 minutes: if they missed a whole day, teachers would notice, and Hermione especially would, too.

"And especially Hermione" makes it sound like Hermione is a teacher rather than a student, so you need to add "too" to exclude her from a formerly mentioned group. A colon would work better here because the second sentence explains why they would go to class instead of seeking help.

It was chaos from step onethe start. It was chaos from the start.

"From the start" works better here: alternatively, you could say "each and every step of the way."

InBoth Charms class likeand Herbology class was a chaosere a mess, since they both didn't know how to act as they usually do. Both Charms class and Herbology class were a mess, since they both didn't know how to act as they usually do.

InBoth Charms class likeand Herbology class was aere chaos since they both didn't know how to act as they usually dolike eachother. Both Charms and Herbology class were chaos since they both didn't know how to act like eachother.

"Chaos" is an adjective, so you don't need an article. Also, saying "Both X and Y were..." sounds much better in this context: "like Herbology class" implies we know how Herbology was, which we don't, because you haven't mentioned it yet. Also, they're still technically "themselves," so you need to use "eachother" to communicate that they couldn't mimick one another.

Finally, Hermione helped them.

Finally, Hermione helped them: she brewed the potion that cured them with easye. Finally, Hermione helped them: she brewed the potion that cured them with ease.

It's obvious you mean the potion that cured them, but you still need to specify exactly what potion you mean. Also, subject repetition is frowned upon in English unless you're trying to emphasize something, and even then it's subject to certain conditions.

They pointed both their wands at the potion. They pointed both their wands at the potion.

They pointed both of their wands at the potion. They pointed both of their wands at the potion.

Again, you haven't mentioned any wands, so you need to specify it's their wands they're holding.

Flash.! Flash!

I think the exclamation mark would help here, to emphasis the fast and strong nature of the flash.

Flash.! Flash!

With this type of language, exclamation marks are usually used (onomatopoeia and such language, for example: "Bang!" "Kaboom!" "Ring, ring!")

They opened their eyes, they. They were back. They opened their eyes. They were back.

I think this sounds better seperated into 2 sentences

They opened their eyes, they back back in their own bodies. They opened their eyes back in their own bodies.

Did you mean "back in their own bodies" here?

At the night, they both thanked each other because they had both learned to be kinder and braver. At the night, they both thanked each other because they had both learned to be kinder and braver.

At theThat night, they both thanked each other because they learned to be kindboth braver and bravkinder. That night, they both thanked each other because they learned to be both braver and kinder.

While not wrong at all, "both braver and kinder" flows better.