Just checking that you are talking about doing your fieldwork now but writing up the dissertation next academic year?
Getting ethical clearance for research with children is essential and has to be done before you set foot in any school or approach any pupil.
My advice is to crack on wth your ethics application asap - but you won;t be able to do that until you work out your research questions and your research methodologies.
A few things to be aware of.
If the subject matter is non-controversial, that will make things a lot easier. Research into a controversial subject is likely to be rejected
The school may be able to give blanket consent but first they will have to check with all the parents. A well designed consent form with things like a Data Protection statement will be required for securing consent.
The questionnaire will have to be produced in advance of applying for ethical approval, and once approved you won't be able to make any changes without going back for reapproval.
There are many more things to consider, they are just some of the important ones.
It is possible to get consent for this kind of research but universities have massively tightened up on any research involving children or vulnerable participants.