In the US, you are required to report ALL income - technically, even if you get $6 for taking a survey. Whether or not you will owe taxes depends on your total income from all sources. It also doesn't matter how many times an employer hires and rehires you - if the total for the year is $600 or more, they are required to file a 1099 to the IRS. They are not required to file for totals under $600 - but you are still technically obligated to report the income on your tax returns.
Does this count if they wouldn't normally be filing a tax return. Remember, this is a student who made some extra cash freelancing (I doubt it makes a difference though, *sigh* bureaucracy). By the way, I'm obviously not a tax expert , but that was what I had heard for general reporting.
You are required to file a return if your net self-employment earnings are equal to or greater than $400, if you are subject to the alternative minimum tax, or if you are a nonresident alien with a tax liability not covered by withholding. http://www.irs.gov/individuals/article/0,,id=96623,00.html
A lot depends on your personal circumstances, like there are religious exemptions, etc. (I am assuming you are in the US) If you never filed taxes before and IRS doesn't have you in their database, I would say don't file. You don't exist for them (as long as no one reported your income to them)
You pay on everything you earn. Then each country has brackets of income tax % wise depending on how much you earn.