My school blocks many random things (like duckduckgo and drifthunters) swearing that they did it because of the Children's Internet Protection Act (CIPA), but that only really says to ban obsene content, child pornography, and to track what they do. I'm curious if this is illegal?
No it is not illegal
A school can ban or block online content from its own computers or connections as its administrators or teachers think proper, and does not need any law that authorizes or requires them to do so.
The US Children's Internet Protection Act (CIPA) requires schools and libraries which receive discounts for Internet access or internal connections through the E-rate program to adopt an "Internet safety policy" This must include measures to
block or filter Internet access to pictures that are: (a) obscene; (b) child pornography; or (c) harmful to minors.
The policy also "must include monitoring the online activities of minors" and must address several other issues, including "access by minors to inappropriate matter". It does not however, require tracking of usage by specific individuals.
But the school may, if it chooses, adopt a policy broader than the requirements of CIPA, which are a minimum standard. The school may block content thought to distract from school work, for example, or that the relevant school authorities think unhelpful or contrary to its goals. This is in no way required by CIPA (unless it is part of addressing the issues CIPA demands be addressed), but is also in no way prohibited, it is the school's choice.
Institutions that adopt a CIPA Internet safety policy must "provide reasonable notice and hold at least one public hearing or meeting to address the proposal." But there is no legal requirement for the school to accurately describe which of its policy items are in fact required by CIPA.
The only penalty for failing to comply with these CIPA requirement is loss of the E-rate discounts. A school may choose not to comply and forgo the discounts. Knowingly providing obscene content to minors might well be in violation of other laws, however.
In short, the school can block whatever it sees fit, for whatever reasons school authorities think proper, unless some other law or constitutional right prevents this. I do not know of any such law or right, although I am researching the matter further. There is not generally a constitutional right to unfiltered internet access in school, or indeed to any access in school at all.
Schools can generally ban whatever they want, but there are some restrictions. The first principle is that public schools have less freedom to ban things compared to private schools: arms of the government cannot infringe on your constitutional rights, but constitutional rights are about the relation of the individual to a government. A state can pass a specific law asserting the right to surf the internet or whatever seem applicable, but there aren't any constitutional provisions guaranteeing that right that I know of (you'll have to name the city and state to get more detailed information).
The most-obviously applicable legal provisions that a ban infringes on is the First Amendment, which is a fairly powerful protection. Setting aside schools, the government cannot universally ban browsing the internet. However, government agencies can ban browsing the internet in certain contexts, for example when you are serving on a jury. A public school can ban web-browsing during class, on the grounds that it is disruptive to the purpose of the class. A private school, however, can completely ban accessing the internet while on school property, as part of the contractual relation between the student's parents and the school.
No person has the right to compel another to break the law. A school must make many decisions as to what the law requires of them, so a school could make a good faith determination that allowing students to do X puts them in legal jeopardy (example: schools cannot allow students to sexually assault other students, since doing so puts the school in legal jeopardy). Some matters are clear cut and others are not, so it might turn out that allowing X would not actually put the school in jeopardy. But the school has to make a judgment, and unless that judgment is manifestly unreasonable, it will be supported by the courts.
The remedy, in the case of an unclear legal risk to the school for doing X, is to hire a really good attorney and sue the school. When the case reaches the higher courts and it is found that the school violated a constitutional right of the student by banning X, thereafter (until countermanded by a higher court) the school will know that banning X is not legal.