It was designed by two young children who were eventually arrested by law enforcement in 1974, and  It was authored by a Zodiac Killer employing a different handwriting technique. So first we will take a look at an article published in the Vallejo Times-Herald on May 17th 1974:
"Two 16-year-old Benicia youths were apprehended by police here as suspects in a series of phoned bomb threats that prompted the evacuation of Benicia High School four times during the day. Officers said the pair allegedly made two phone calls to the Benicia Fire Department, warning firemen that two bombs had been planted at the high school and were going to explode. Police said the youths allegedly made two other phone calls to the school, informing officials two bombs had been planted - one inside and the other outside the building. In each instance, police said, the 700 high school students and faculty were evacuated and a thorough search of the building made for explosives. Poilice said the two youths are Benicia High School students and were apprehended near a payphone on J Street, between West Fifth and West Sixth Streets. Officers said the youths admitted making the bomb scare calls from a phone booth but gave no reason for doing so. Police said both were lodged in Juvenile Hall in Fairfield on felony charges accusing them of phoning bomb threats. Officers revealed Benicia High School has been plagued with a series of phoned bomb threats since the start of Fall term last September. During one two week period, police said, school authorities received one or two such calls every day. Police said two 14-year-old boys were arrested as suspects about a month ago (approximately April 17th 1974), admitted making the phone calls and now are on probation".
This article in the Vallejo Times-Herald, coupled with the Benicia High School note (below) could give us the exact date the threatening note was directed toward the school, assuming these two youths were responsible for both. There is an extremely high probability this was the case - and if so - the Benicia High School note was likely placed at the school in the September of 1973. The next thing to do is pinpoint an exact date using the note itself.
"I'm sorry, but by Sunday I shouldn't have said anything about what time I was to blow up the school, so instead I will do it between now and the 22nd December. So, I have killed by Sunday and you will find him (the body) up by Lake Herman Road. If you get there in time you will find him before the dogs eat him. I hope they do".
The phrase "This his blood" has been fashioned with the S's reversed, possibly indicating that the alleged murder had taken place on Saturday 22nd September 1973 - in other words, by Sunday. It is then easy to subconsciously write December 22nd 1973 as an end date for blowing up the school, as this date is already in your mind for September (and a nice round figure of 3 months to project forward from and arrive at December 22nd). A suggested murder committed on Saturday September 22nd 1973, with the note being placed at Benicia High School later that day or the following day, and hence the past tense of "you will find him" and get there "before the dogs eat him". Mischievous kids with too much time on their hands at the weekend, depositing the note at the high school. It is my guess that the note was likely authored on the Saturday and placed at Benicia High School later that day - on September 22nd 1973.
Then we have to consider the chance that it was authored by the Zodiac Killer. On the face of it, the communication appears to be inspired by previous Zodiac correspondence, with the threat of a bomb, the reversed letter N on the first line mimicking the Halloween Card, the Confession Letter "about time" and the reference to Lake Herman Road, with the near date of December 22nd a reminder of past events. The only way to confirm the authorship of this letter as being written by the hand of the Zodiac Killer, would be to draw parallels to previously unreleased Zodiac material by 1973. The December 16th 1969 Fairfield Letter is the only correspondence to exhibit any promise, with a comparison of "blood" to "bleeding", the knife-like shape smeared across the Benicia note, the droplets beneath the knife in both instances, and the positioning of the pseudonym "Zodiac" toward the right side of the blade-like design.
Other than this cursory observation, it simply depends on whether you believe that two Benicia High School youths were responsible for the note, having previously made numerous bomb threats upon the school, or the Zodiac Killer had reappeared and threatened Benicia, four years after the brutal murders of David Faraday and Betty Lou Jensen on December 20th 1968. I shall take the first option, but the decision is yours.