Background. "T4" is the UK's term for a student visa, it's short for "Tier 4 of the Points Based System (PBS)". This visa is arguably the most abused in the PBS and the government is continually tightening the rules. Schools have become extremely sensitive about compliance because loss of their PBS license means the loss of an important source of revenue. There are rules for both the institution and the the students. One of the rules for the institution is that they must have an "Authorising Officer (AO)" in place who is responsible for T4 compliance, more about this later.
One of the rules for students is a time constraint on how early a person can arrive in the UK prior to the first day of classes. It's not worth going in to the rationale behind this rule except to say that it's meant to reduce the improper use of the visa (i.e., abuse). Non-visa nationals who obtain a T4 like to work around this rule by using their visa-free status to show up as a visitor; visa nationals also like to work around the rule by combining their T4 with a visitor visa (thereby allowing them to enter in advance of their T4 start date).
Both of these cases may be discovered at the control point when the student presents his passport during the landing interview. When this happens the Immigration Officer has the option, but not the obligation, to call the school's AO and ask if he is aware that the student is arriving outside of the permitted arrival dates. If the AO says 'yes', it's been discussed with the student and their grounds are reasonable, then all is well.
But if the AO goes "HUH?", then both the school's license is jeopardised and the student is in trouble with UKVI. So to clear the air with UKVI, the AO will often say that the student no longer qualifies for the given course of study (or some other similar reason). For the student, this means their visa is revoked and and they get bounced.
Another strategy comes up when the student uses their visa free status to enter as a visitor and then channel hop when their T4 starts. Even even if this works, they can still be done later as an illegal entrant. They like to turn up the magnifying glass if a student several years later applies for a T2 (work permit).
In short, these strategies are adventurous. Abuse is something that gets IO's deeply annoyed and causes serious upset with the British public. End of background.
Your question is...
I'll arrive in August under the tourist visa, leave to travel in
Europe under a Schengen visa, and then return to the UK, arriving
under my Tier 4 visa. Are there any problems with this plan?
What you propose to do works "on paper". But it also conforms to a known pattern of abuse. Even if you are well-intended, the IO has the right to turn up the magnifying lens and find a problem.
So the overall, generic answer to your question is: get permission from the Authorising Officer beforehand. Preferably written.
