If you want to put a note without a \item use para option in the tablenotes environment.
Moreover:
- Look at When should we use \begin{center} instead of \centering?
- avoid using
resizebox to resize a table, it is better to use the appropriate font, for example \large (but in this case the normalsize caption is awful, in my opinion, I wouldn't change the font at all).
- since you loaded
dcolumn, use it for better number alignment.
I have also loaded array to add some space between rows modifying \arraystretch.
\documentclass[portrait 12pt]{article}
\usepackage[left=1in,top=1in,right=1in,bottom=1in,nohead,nofoot]{geometry}
\usepackage{array}
\renewcommand{\arraystretch}{1.2}
\usepackage{booktabs}
\usepackage{lscape}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{booktabs}
\usepackage{dcolumn}
\newcolumntype{d}[1]{D{.}{.}{#1}}
\usepackage[normalem]{ulem}
\usepackage{rotating}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{multicol}
\usepackage{hyperref}
\usepackage{float}
\usepackage{adjustbox}
\usepackage[utf8]{inputenc}
\usepackage{verbatim}
\usepackage{color}
\usepackage{caption}
\usepackage{amsmath}
\setlength{\parskip}{.25cm}
\setlength\parindent{0pt}
\setcounter{tocdepth}{1}
\newcommand{\sym}[1]{{#1}}
\title{Tablet Data Report}
\author{Cory}
\date{}
\def \OutputPath {"C:/Users/ifmr/Dropbox/Education v2/Clean Data Pipes, 2018/Analysis/launcher/Output/"}
\begin{document}
\maketitle
\section{Summary Statistics}
\begin{table}[H]\centering\large
\caption{Cumulative Session Length (Hours)\label{alabel}}
\begin{threeparttable}
\begin{tabular}{l*{5}{d{1.2}}d{2.2}}
& \multicolumn{5}{c}{\textbf{Class}} \\
\cmidrule(lr){2-6}
\textbf{School}&\multicolumn{1}{c}{\textbf{1}}&\multicolumn{1}{c}{\textbf{2}}&\multicolumn{1}{c}{\textbf{3}}&\multicolumn{1}{c}{\textbf{4}}&\multicolumn{1}{c}{\textbf{5}}&\multicolumn{1}{c}{\textbf{Total}} \\
\midrule
Asharfabad&2.13&1.87&2.28&2.95&2.56&11.78 \\
RN3&0.24&0.00&0.00&0.00&0.63&0.86 \\
Seiko&0.00&0.00&0.00&0.00&0.75&0.75 \\
\bottomrule
\end{tabular}
\begin{tablenotes}[para,flushleft]
If your table doesn't have a note, why do you use threeparttable?
\end{tablenotes}
\end{threeparttable}
\end{table}
\begin{figure}[H]
\centering
\includegraphics[width=0.6\linewidth]{example-image-a}
\caption{Average Session Length By Week}
\end{figure}
\end{document}

Edit:
Since you can't change your input file, the best alignment, in my opition is r:
\begin{filecontents*}{cumulative_length.tex}
& \multicolumn{6}{c}{\textbf{Class}} \\
\textbf{School}&\textbf{1}&\textbf{2}&\textbf{3}&\textbf{4}&\textbf{5}&\textbf{Total} \\
\midrule
Asharfabad&2.13&1.87&2.28&2.95&2.56&11.78 \\
RN3&0.24&0.00&0.00&0.00&0.63&0.86 \\
Seiko&0.00&0.00&0.00&0.00&0.75&0.75 \\
\end{filecontents*}
\documentclass[portrait 12pt]{article}
\usepackage[left=1in,top=1in,right=1in,bottom=1in,nohead,nofoot]{geometry}
\usepackage{array}
\renewcommand{\arraystretch}{1.2}
\usepackage{booktabs}
\usepackage{lscape}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{booktabs}
\usepackage{dcolumn}
\newcolumntype{d}[1]{D{.}{.}{#1}}
\usepackage[normalem]{ulem}
\usepackage{rotating}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{multicol}
\usepackage{hyperref}
\usepackage{float}
\usepackage{adjustbox}
\usepackage[utf8]{inputenc}
\usepackage{verbatim}
\usepackage{color}
\usepackage{caption}
\usepackage{amsmath}
\setlength{\parskip}{.25cm}
\setlength\parindent{0pt}
\setcounter{tocdepth}{1}
\newcommand{\sym}[1]{{#1}}
\title{Tablet Data Report}
\author{Cory}
\date{}
\def \OutputPath {"C:/Users/ifmr/Dropbox/Education v2/Clean Data Pipes, 2018/Analysis/launcher/Output/"}
\begin{document}
\maketitle \section{Summary Statistics}
\begin{table}[H]\centering\large
\caption{Cumulative Session Length (Hours)\label{alabel}}
\begin{threeparttable}
\begin{tabular}{l*{6}{r}}
\toprule
\input{cumulative_length}
\bottomrule
\end{tabular}
\begin{tablenotes}[para,flushleft]
If your table doesn't have a note, why do you use threeparttable?
\end{tablenotes}
\end{threeparttable}
\end{table}
\begin{figure}[H]\centering
\includegraphics[width=0.6\linewidth]{example-image-a}
\caption{Average Session Length By Week}
\end{figure}
\end{document}

\begin{tabular}{4*{6}c}. How many are your actual columns? – CarLaTeX Aug 10 '18 at 04:49