Any advice,please? I need help!

I'm in a similar situation to this.
you need some time to be on your own without any disturbances to come to terms with everything. you need to understand that your family were worried about you being upset about the dog, so thats why they didnt tell you, although personally i think they should never have lied to you. you need to, when the time is right, talk to the most trusted member of your family about your feelings and everything, and calmly ask why they lied to you. there's probably a good reason, and get things out in the open and cleared up.

A lot of bad things have happened to me too recently so i know what you mean about the pretending everythings ok at school, because that seems like the easiest way. maybe if you have a really trusted best friend you could tell her how yu feel, or even your favourite teacher if they are nice. I've felt the same as youve described since my special cat died too- a couple of months ago, which was partially my fault and i feel terrible about it and cry all the time because of the other pressures too. I am planning in the next few months to get a new rescue kitten to make me feel better and to keep my other cat company. maybe you could, in a few months after everythings settled down go out to choose your own rescue puppy, and youd feel better because you were giving an animal a home. you should talk this through with your family and tell them how much it would mean to you.
you should also keep a locked diary of your current feelings or a password protected word document- you'd be surprised how much easier it feels just to have it all on paper instead of clogging up your mind.
You should also get some hobbie to take your minbd off things- maybe look in the back of tabloids for hobbies or youth clubs, or your school bulletin / noticeboard for after school clubs. or even arrange a day out or a sleepover with your friends and tell them some things about how you feel then with out the pressure of school.

If you do all or some of these things, hopefully you will start to feel better. Hopefully you will feel a bit better soon x

Lottey<3 XX