Can a student bring a 3.5in knife to school?
CO Rev Stat § 18-12-105.5(1) says that
A person commits a class 6 felony if such person knowingly and unlawfully and without legal authority carries, brings, or has in such person's possession a deadly weapon as defined in section 18-1-901 (3) (e) in or on the real estate and all improvements erected thereon of any public or private elementary, middle, junior high, high, or vocational school or any public or private college, university, or seminary...
but there are exceptions such as presenting an authorized demonstration, or for some authorized extracurricular reason. The meme about under 3.5 inches derives from the general statute pertaining to concealed weapons, where CO Rev Stat § 18-12-101 defines "knife" as being over 3.5 inches long (and it is a misdemeanor to carry a concealed knife). We'll come back to this.
The applicable part of the definition of "deadly weapon" in the school-specific law is
(II) A knife, bludgeon, or any other weapon, device, instrument, material, or substance, whether animate or inanimate, that, in the manner it is used or intended to be used, is capable of producing death or serious bodily injury.
This raising a question that I encourage you not to test, whether the school deadly weapon definition excludes knives that are longer that 3.5 inches. The definition section for Article 12 starts by saying
(1) As used in this article, unless the context otherwise requires
so that means that "knife" is redefined in a special way (in terms of length), and that is for the entire article which includes the school weapon section. The actual definition of "knife" in the definitions section of Article 12 is
(f) "Knife" means any dagger, dirk, knife, or stiletto with a blade over three and one-half inches in length, or any other dangerous instrument capable of inflicting cutting, stabbing, or tearing wounds, but does not include a hunting or fishing knife carried for sports use. The issue that a knife is a hunting or fishing knife must be raised as an affirmative defense.
The modifier "with a blade over three and one-half inches in length" might be thought to refer to all of the preceding terms, or just the last one – stiletto. These is a rule of legal interpretation that a modifier should be interpreted as referring to the last antecedent, meaning that the length limit pertains to stilettos. The punctuation (lack of comma) supports the interpretation "stiletto with a blade over three and one-half inches in length", meaning that a knife is a knife. Whether or not a literal knife with a blade under 3.5 inches long constitutes a redefined knife, it certainly constitutes a "dangerous instrument capable of inflicting cutting, stabbing, or tearing wounds".
In the context of a specific law restricting the possession of weapons (concealed or not) in schools, the context requires that the definition explicitly mentioned in 18-1-901(3)(e), which is outside Article 12, is being referred to. The reason the law was explicitly written to refer to that definition was so that the general definition in article 12 would be overridden.
My point is that this is a not atypical example of the problem of legal definitions, which can get rather convoluted. If someone has told you that a knife under 3.5" is legally not a knife, you probably now know why they would say that, but also you should know what that's simply not true when it comes to weapons in school.
I omitted one complication, which in this instance is not applicable, namely the kirpan exception. In Singh v. Thompson, 36 F.3d 1102, it was ruled that a prohibition against knives (in school), in that instance, violates the Free Exercise clause (the kirpan must be worn at all times). On the other hand, TSA does forbid taking a kirpan into the secure zone, a matter which has so far not been litigated.