Some Mac users (including me) were able to fix this just by going to the console and typing "R CMD javareconf" and restarting R.
Windows users seemed to have much more significant issues, however. There seemed to be several issues, but here's a solution that helped several of the students, courtesy of Hirzi Luqman:
* installing *source* package ‘ENMTools’ ...
*** moving datasets to lazyload DB
** byte-compile and prepare package for lazy loading
Error : .onLoad failed in loadNamespace() for 'rJava', details:
call: dyn.load(file, DLLpath = DLLpath, ...)
error: unable to load shared object '/Library/Frameworks/R.framewo
Referenced from: /Library/Frameworks/R.framewor
Reason: image not found
ERROR: lazy loading failed for package ‘ENMTools’
* removing ‘/Library/Frameworks/R.framewo
Installation failed: Command failed (1)
ENMtools/rJava was looking for (and couldn't find) the library "libjvm.dylib" in the directory "/Library/Java/JavaVirtualMach
Solution: I just copied the same file from the Java 8 directory (or whatever Java version is installed on the computer; in my case the directory was "/Library/Java/JavaVirtualMach