该句子改为否定句是：I haven't a new school bag.
英语句子改成否定句的方法：在动词后面加not，有is、am、are的句子会在这些词后面加not，I am(not)a boy。没有is、am、are的句子会在动词前加don't或者do not。如果有must、may、can这类的词，在这类词后面加not。根据句子的不同情况进行改动。
否定句是表示否定的句子。必须有否定词，古代汉语中否定词可以是副词“不”、“弗”、“毋”、“勿”、“未”、“否”、“非”，可以是动词如“无”。
否定句分类：
1.一般否定句
I don't know this. No news is good news.
There is no person（smoking）/not a person/not any person（smoking）in the house.
2.特指否定
He went to his office, not to see him.
I am sorry for not coming on time.
I don't think/believe/suppose/feel/imagine you are right.
3.部分否定
All the answers are not right
All is not gold that glitters
I don't know all of them.
I can't see everybody/everything.
Both of them are not right.
4.全体否定
None of my friends smoke.
I can see nothing/nobody.
Neither of them is right.
Nothing can be so simple as this.
5.延续否定
You didn't see him, neither/nor did I.
You don't know, I don't know either.
He doesn't know English, let alone/to say nothing of/not to speak of（更不用说）French.
6.半否定句
We seldom/hardly/scarcely/barely hear such fine singing.
I know little English. I saw few people.
7.双重否定
You can't make something out of nothing.
What's done cannot be undone.
There is no sweet without sweat.
No gain without pains.
I can't help/keep/laughing whenever I hear it.
No man is so old but（that）he can learn.
8.排除否定
Everyone is ready except you.
He did nothing but play.
But for your help, I couldn't do it.
9.加强否定
I won't do it at all.
I can't see it any more.