I am just wondering if this is an acceptable approach to write an opinion essay? As you may find that I put forward some reasons to agree and then more reasons to disagree. At the end, I concluded with the disagreement part of it.
.... this is alright. You can give reasons for this argument in one para and against in the next one. Then express your opinion in the conclusion. However, this is the difficult way of tackling this task within the given time. It's not wrong and certainly acceptable, but time consuming :)
When I started writing the essay, another approach also jumped into my mind and that was about stating my position in introduction and writing the rest of the paragraphs just to support my opinion.
... I vote for this because then you know what you need to do in the rest of your essay. Not only you, even the examiner would know what he's going to follow in the next parts. So, this approach would be more helpful for you to manage time and present a neat essay :)