"I think John will come to school today."
Possible reason is that John is the speaker's son or brother. When you refer your own family members to someone outside you do not add さん because it is impolite.
hmm no way to tell via the sentence. hm.. heh, I thought it was just in refering to yourself that you omit the -san but family too eh? k. thanks
Placing 「今日」at the beginning of the sentence means that on this day you have the thought that John will come to school as opposed to having that thought yesterday.
It's a bit nuanced but it becomes clear once you use two temporal adverbs in the same sentence: Today I think that Jon will come to school tomorrow.
while bmpurifoy's explanation is great, Duo accepts "今日” at the beginning of the sentence. April 26, 2018.