ページを開きましたって英語でなんて言うの？

So, you may have the page open already in preparation and the teacher asks you to go o that page. At this stage you inform your teacher that you are already on that page - or "Yes I have that page open already." If you are searching for some and time and then find the page, you may inform the teacher: "I'm there now!"

"I have opened it." or "It is open now." These are ways you can notify the teacher that you have opened the page.
For example:
Teacher: Can you please open the link I have send you?
You: I have opened the link now.

"Ok, I opened it." can tell a teacher that you are ready when you have opened a website or book.

All of these would be a good way to explain that you are ready and have the page/material open
so 'I have opened it', 'I have the page open ready' and 'I have opened the page that would let the teacher know that you are ready to start the lesson

When you want to let your teacher know that you already have opened a link that they sent you in the chatbox on Skype, then you may let them know in the following ways:
-I have it open
-I opened it
-I already have it open

"It's open."
In this expression you are answering directly to the question. This is the most popular form of answer to this type of question.

"Done!"
With this expression you are letting the person know that you are done with the task they told you you had to do. In this case your teacher told you to "open your textbook to a certain page/webpage" so you reply with "Done!" confriming that you have done what was requested.