Chris,
First of all, thank you for your response. I have been looking into the file you wrote about in your last message, and it seems that your suggestion is very useful and a good idea.However, I am not allowed to access into the school server, and it seems pretty difficult to convince the responsible staff to do this, and the process will last months (I am still pressing for the installation of STACK since June, so I am working by now in a local server temporaly and preparing the exercises for this year).
I have come up with a not-so-good solution that I send you in case you want to see it. (I am sure it is not the best material).
I have duplicated all the variables, making a string version of all of them with the modifications I want to be shown. Also, I have used the function ascii to avoid the prohibition of using the character ":". I have substituted the dot character by "." as it seems than the ascii character is not recognized by the database and turns out an error message.
So, not a very clean solution, but it works...
Best regards and thank you again!!
Eduardo