As far as I'm concerned, this is no difference. 了 dose not only mean past tense(我吃了饭）, but also it means future tenses（我要吃饭了）.

在中文语法中，“了”一般用作语气助词。 Generally speaking, this word makes your oral expression more kind and comfortable, and you also could find other words used in similar way, such as '吧' '啦' '呀'...

The difference is that the one without 了, is a matter-of-fact statement and can be translated as, "This student and I have not seen each other for a long time." With 了, it is an indicator to soften the tone to come across as more polite or without the speaker carrying some kind of grudge or resentment, "Ah, this student and I did not see each other for a very long time."

First, the second one is more usually uesd in daily life and it is easier to make conversation natural. Second, 了 have the intimacy of feeling.

In China, most people are prefer the second one addind了 when they talk. The only purpose is to make it sound more fluent and not that harsh.