? about external filtering and proxy server

hello and thanks for reading this,
i am a windows user who just install apache.
it is really easy to configure.
i was able to get to the proxy server to work within minutes.
i was able to get the external filter module to work only when the proxy
server functionality was turned off.

is it possible to run the external filter and the proxy server at the same
time?

i want to help a school to create a poor-mans proxy server and logger and
filter.

how would i do the following:
when a proxy client connects to the proxy server, some kind of program would
run for each request.
the program only needs the requested url. then this program decide what to
do and would return the same url or another redirected web page local on the
proxy server

is this possible?

thanks,
dave