Chapter 1 review:
Very interesting premise. I love the idea of a Pokemon virus going about. I can see so many ways this can branch off!
First off, I think a bit more showing and less telling would really enhance your story. From the first few lines:
It was a greyish afternoon in early October. Rose, Jason and Puck had finished working on their assignment and were chatting as they walked from school towards the metro station. It was Friday, and they’d just finished their group assignment after class was dismissed.
You tell us Rose, Jason and Puck are chatting. They finished working on their assignment. It sounds like a summary, and that's something you should try avoiding in the future. Telling us they're chatting is redundant if you can show us the chat (which you did right after this paragraph).
When you indent paragraphs, do so whenever a new dialogue starts.
'I’m glad we finally got around to finishing the assignment!' Jason said, and he stretched his back. Jason was always very relieved whenever he finished something important.
'Yeah, although it’s not like we had that much to do. We just kept procrastinating!' said Puck, and she laughed. Puck was never afraid to state her opinion. <- again, too much telling. She's never afraid to state her opinion, just show that instead of stating it.
She threw the pokéball up in the air and after a flash came out of the ball, a small yellow Pokémon with black patterns and pink cheeks appeared.
Awkward. I'd fix it up as: She threw the pokéball in the air, and a small yellow Pokémon with pink cheeks and black patterns on its ears appeared after a flash.
‘Uhu,’ Puck agreed as well. ‘Dale’s meeting up with us after work right? Which is when?’
‘He works in a café at the corner of my street, he’ll be with us by 7.30 7:30, you mean? 30, should be a comma, be consistent with your grammar’ Rose answered.
Be consistent with tenses and grammar. If you are in past tense, stick to past. Remember that a story is not just a summary. You're relaying the events. Examples:
Puck was in shock.
It was a very brave and energetic Pokémon, and would keep the Sandslash away with great ease.
...And so on and so forth. You mentioned English isn't your first language so for a first fanfic, despite all my nitpicks, I'm honestly impressed. You have good ideas, as well!
I love the action you have going here. Sandslash on a rampage, wooo.
There's a lot going on here and it's actually very exciting. The way you choose to tell the story, though, can pull readers in even more. I want to feel the terror of having to see Sandslash cutting through people like paper, the fear when you realize there are more coming after the first one. A bit more description would also help, but like I said, despite my nitpicking, I do like the plotline and you leave us hanging for more, which is always a good thing.
Very interesting, I will read more in the future. ^_^