YOU SAID:

I can't come to school today! I have a serious headache.

INTO JAPANESE

私は今日学校に来られない!深刻な頭痛があります。

BACK INTO ENGLISH

I do not come to school today! have a severe headache.

INTO JAPANESE

私は今日学校に来ていない!激しい頭痛があります。

BACK INTO ENGLISH

I did not come to school today! have a severe headache.

INTO JAPANESE

私は今日学校に来なかった!激しい頭痛があります。

BACK INTO ENGLISH

I did not come to school today! have a severe headache.