How to reduce to an NP-hard problem?
Just an extended comment.

I think that before trying to find a reduction, you should try to better formalize the problem:


define exactly what the parameters of the problem are, formalize their constraints, and formalize the question;
try to simplify (or remove) the details;


For example: 


If a group consist of 1 student, this conversation last 10 minutes. $\rightarrow$ it is equivalent to a group of two students, so you can remove it;
Timespan is 13.00-19.00 $\rightarrow$ in the other rules you use minutes, so convert the "timespan" to $T$ minutes;
try to think if a simplified version of the problem has a quick solution or it can be still hard (e.g. if the event last only one day rules 7 and 8 can be dropped)
express the rules with a math formula: e.g "if an elementary group $E$ of $n$ students talk with a high school group $H$ of $m$ students then the total time $TT$ required is $TT(E,H)=....$"; "the total max wait time $WT$ for group $E$ is $WT = ...$"


Furthermore there is a point that it is not clear (to me):


suppose you have an elementary school group $E$ having $n$ students that talk with a high school group $H$ having $m$ students; what is the total time required? (1) $5 * n$ (the talk is public) or (2) $5 * ( n / m)$ (each student of E talks with a single student of H). If the talk is public, then the number of students is redundant, and you can think only in terms of total time required by a school group.