Addressed in the NFHS Rule Book (can purchase online if you want) this is what you are looking for:
Rule 4-5-9
Art. 8 It is an unfair act when a competitor receives any assistance from any other person. Assistance includes:
a. Interference
b. pacing by a teammate or persons not participating in the event
f. coaching a competitor from a restricted area
There is further clarification in the case book with some examples, the best being the following two:
rule 4.5.8 situation i:
During the 1600 meter run, A2's teammates are stationed along the edge of the backstretch or curve and are: (a) calling out lap times; (b) running along the side of the track trying to get A2 to run faster; or (c) cheering or offering encouragement to A2. RULING: In (a) and (b), A2 is subject to disqualification. In (c) this is legal provided the teammates are not in a restricted area.
rule 4.5.8 situation j:
During the 1600 meter run, a teammate of A1: (a) runs a few steps toward the track shouting encouragement, or (b) runs alongside A1 for five or more strides. RULING: Legal in (a). Illegal in (b). Comment: It is not the intent of the rule to disqualify anyone for a spontaneous act of enthusiasm. However, running alongside a competitor for five or more strides would be considered pacing.
It would ultimately be a judgement call on the part of the meet referee and would be a DQ if the meet referee felt that an unfair advantage was given to the athlete.
Bottom line: Cheer from an unrestricted area (outside the oval) and feel free to give splits anywhere you please from an unrestricted area. Whoever said you can't yell splits is incorrect, you can, so long as it's in an area that any coach can do that from. You may not use a whistle or something like that in lieu of verbal instructions to inform a runner of their pace.