I'm going to take a stab at your first question. If you think my reply is taking away the teachers' attention, you can just bump up the thread by writing a reply with any word.
'Be warned' and 'attention' are different.
"Now everybody, attention!" = "Can I have everyone's attention, please?"
You'd use this before you speak to a group of people. You want them to stop whatever they are doing and pay attention to you. It's also a military posture.
"Now, be warned."
Be warned of upcoming something; danger, surprise or what have you.
I may use it in this format: "Now, be warned. You chose a field many people virtually die trying."
Oh and this is just my wild guess, but 'attention' in French probably means 'warning' in English.
I'll leave the second question to the teachers in this forum. Please feel free to add on to, correct or bash my post. No input will go unappreciated.