Setup: Non-rated elementary tournament, already decided to use USCF tie-breakers for individuals.
Kids also compete by grade level as a team (3 per team per school). If two teams tie for the ‘top’ team, what tiebreakers should be used.
I think the same three for individual since this factors in the ‘strength’ of the competition etc. (MM, Sol, Cum)…
Is there anything in the actual rule book to dictate?
Advise?
Hold a playoff and don’t subject the players to the mathematical arbitrariness of tie breaks!
Not possible to add additional rounds… five round Swiss… no more.
So… given that a team can tie… are the three USCF individual tiebreaks (MMed, Solk, Cumulative)… does the rule book state to use the same? I do not have a rule book to look 
Where to order a copy of the USCF Rulebook:
You can also use the Team Tie Break as stated in the USCF National Scholastic Chess Tournament Regulations.
1.4 All state scholastic organizations are encouraged to adopt these regulations for state scholastic championships.
The regulation is available for download at:
Modified median is bad because it discards different numbers of opponents and different opponents depending upon the final score.
Consider a 5-round 4-player team event with teams tied at 10 points. One team with all players 2.5, lose the high and low scores of opponents for each player. So it sums up a total of 12 opponent scores, largely in the middle. In comparison, if the scores are 4,3,2,1 the 4 and 3 lose only a bottom score and the 2 and 1 only a top score, so it sums up 16 opponent scores.
The rule book recommends Modified Median, Solkoff, Cumulative, and Cumulative of Opposition for individual tie breaks. It does not mention team tie breaks for combined individual/team tournaments, which implies that the same ones could/should be used. As Tom Martinak pointed out, the Modified Median is not valid for a team (although it is sometimes used). WinTD automatically omits it from team tie breaks and uses the remaining three.
The only recommendations for teams that I have been able to find in print are:
USCF Rule Book (omitting MM): Solkoff, Cumulative, Cumulative of Opposition
USCF National Scholastic Regulations: Cumulative, Solkoff, Sonneborn-Berger
There was a thread in the Tournament Direction forum a while back about why these recommendations are different, but there was no consensus. The rule book recommendations seem more logical to me, but you just have to take your pick. As Mike Nolan said, all tie breaks are arbitrary. But they are a necessary evil in many scholastic tournaments.