Learn Portuguese (Portugal): O comboio parte de Lisboa com setenta alunos. - The train leaves Lisbon with seventy students.

No. Even though setenta (seventy) ends in the letter a, it does not change based on gender. The tens words from trinta to noventa are invariable, meaning they stay exactly the same whether you are counting male students (alunos) or female students (alunas).

Why is it de Lisboa instead of da Lisboa?

Most cities and towns in Portuguese do not take a definite article. Because there is no article before the city name, the preposition de (from) stays exactly as it is and does not merge into da.

What verb does parte come from?

It comes from the regular verb partir (to leave or to depart). Because o comboio (the train) is a singular noun, we use the third-person singular form, which drops the ir and adds an e to become parte.

I have seen the word trem for train before. Why is it comboio here?

In European Portuguese, the word for train is always comboio. You will see trem in Brazilian Portuguese resources, but it is not commonly used in Portugal.