There have been 3 official ISO C standards, published in 1990, 1999, and 2011. There have also been several "Technical Corrigenda", official documents making corrections to the standards.
The standard itself can be purchased in PDF (or paper) format from ISO or from your national standards body, such as the US ANSI. ANSI sells the C11 standard for about $30, but with strict licensing requirements. The Technical Corrigenda are available by themselves at no charge, but they're not useful without the standard. If you want to make the standard available to your class, that's probably not a practical way to do it.
But drafts of the C standards are often made available on the ISO C committee's site.
N1256 is a draft of the C99 standard with the three Technical Corrigenda merged into it. It's actually better for most purposes than the official C99 standard.
N1570 is the most recent (as far as I know, as I write this) draft of the 2011 ISO C standard, published before the ISO standard was released. There are only a few minor editorial differences between N1570 and the official standard.
There's also a small Technical Corrigendum to the C11 standard, containing a couple of fixes that aren't in either N1570 or C11: due to an oversight, the published standard didn't define the value of __STDC_VERSION__
and the optional __STDC_LIB_EXT1__
macros properly (both are 201112L
).
Each standard (and draft) has a summary of the differences between it and the previous standard.
Apart from the standard (which is not really written for general programmers), Harbison & Steele's C: A Reference Manual is a very good reference. The current 5th edition covers C90 and C99, but not C11. (I don't know of any plans for a 6th edition.)